One of the most fun aspects of fixing up an old car is making visible changes and improvements. Rebuilding the chassis or brakes is important, but who's gonna know? It's good to mix in some mostly cosmetic improvements to keep your spirits up. One of the most noticeable changes are tires and wheels. That old bomb probably needs some new shoes and there is so much to choose from at swap meets, on the web and in the wrecking yards. Tires are expensive. I have found that used tires are selling for around 30-35 dollars at used tire stores. It's a good idea to price the tires that you want, to determine if the tires offered up are really a good deal. If you can find a nice set of wheels that still have a good set of tires already mounted then you are ahead of the game.
If your car is a popular, fairly late model or has current incarnations you will have a lot of options. If your car is older or a little offbeat you are going to have to really keep your eyes open. I thought that I would have all kinds of choices available since I had an early Mustang. While the lug pattern is shared with the current generation there are substantial differences due to the rim diameter, width and especially backspacing. Looking back at the 2005-2015 models these wheels are too wide, (many are 8in.) too big ( 16-19 in.) and have way too much backspacing. The 1994-2004 models have similar fitment problems and don't even interchange with each other. The 1979 -1993 models had four lugs instead of five and are totally unsuitable. There weren't a lot of low cost options.
I had a similar problem with my 280Z. These were 4 lug 14 in. wheels. I already had a good set of stock steel rims and wheel covers but trying to upgrade to 14 in. or especially 15 in. alloys was a real challenge. The later 280ZX turbo models came with a couple of different 14 in. alloy designs. There is the "swastika" and "laced spoke " designs. I was also able to find a set of 15 inchers on CL. Missing the center caps, of course. I came up with a low buck center cap replacement that will work for many wheels.
First you need to determine the wheel lug pattern and spacing. There quite a few on line sites that will list the lug pattern and you can check the listing for possible interchangeble replacements. The Tire Rack has a great site that has a tremendous amount of info.Usually there is a family of cars that will interchange. With early Mustangs platform mates such as Falcons, Cougars, Fairlanes etc. are possible sources.
The internet is a great source of information. There are enthusiast forums for almost every kind of car. Search the site for posts that discuss interchanging wheels. Using this method I learned that 16 in. wheels do not work with early Mustangs. I also learned that Ford Ranger and Explorer wheels were an inexpensive 15 in. wheel option.
Pay particular attention to backspacing. The wheel has to clear the brake drum/rotor caliper assembly and the ball joints and tie rod ends. Also pay attention to wheel lug length. Replacing a steel rim for another is straightforward. Aftermarket alloy or custom wheels will require the proper lug nut. Some of these will be difficult or expensive to obtain, best if they are included with the wheels. I found that the OEM (Ford Ranger) alloys that came on my car had the proper backspacing and used standard taper style lug nuts but the mounting face of the wheel was thicker than the original steel wheel and the lugs did not protrude far enough to secure the lug nuts properly. I could have replaced them with longer wheel lugs, but I didn't want to deal with that problem at the moment. If you are planning to run wheel spacers ( not recommended) consult a good tire store for advice.
The most obvious improvement in handling will come from the increased tire width and lower cross section. The width of the rim must be compatible with the width of the tire for proper mounting. There is usually a range of a couple of inches, but the wider end of the range will contribute to a more stable sidewall and avoid that "balooned out" look. If you select the right size tire/ wheel combo you can avoid having to roll the fender wells for clearance. Back in the day you would "jack up" the rear with longer spring shackles or air shocks. None of this made for a better handling machine and were many times unsafe. If the car is lowered about one inch handling and appearance can be improved provided that the wheel has proper clearance.
I have bought sets of wheels with good tires still on them. This can save money and headaches because you can try them on to see how the package fits. Of course you have to consider the price the seller is asking. You have to decide how much the rims alone are worth to you, then how much the tires are worth. Lots of times the tires are referred to as "rollers" and they are only good enough to roll the car around the garage or drive to the tire store. The price should reflect the value of the wheels alone. Price out the tires you want, so that you can make an informed decision.
Inspect the condition of the wheels and tires. Check for bent rims, curb rash, cracks and elongated or damaged lug holes. Roll the wheel on flat pavement and see if it rolls true or pulls to one side. I've seen tread separations that were clearly apparent, with one side of the tread surface clearly higher than the other. Look for cracking and checking of the tire sidewalls. It's also really important to check the manufacturing date of the tire. In the last twenty years of so tires have had a DOT number incorporated in the sidewall. This is not a serial number but a batch number.Tires manufactured since 2000 will have the last four digits specify the week of the year and year of manufacture. For example, 5110, 51 indicates the 51st. week of the year, while 10 corresponds to 2010. Tires manufactured prior to 2000 will have the same info conveyed in the last three numbers. For example 239. The first two numbers indicate the 23rd week of the ninth year of the decade; 1999.
Hubcaps and wheel covers are a great low cost way to improve the looks of your car. Most standard steel rims will accommodate almost any hubcap of the same rim size. There are stores that specialize in hubcaps only, but you probably won't find the best deal there. Wrecking yards are a good choice but be sure you find a full set. You can probably find a better deal at a swap meet or on Craigs list. OEM hubcaps are very well made, usually out of stainless steel and high quality die cast chromed pieces. They can be refurbished easily and look great. After market hubcaps are made of lightweight poorly chromed steel or plastic. I don't think that these have as satisfactory an appearance. If a cheap aftermarket cap will fit, so will a much better built OEM.
One of the nice things of going the lowbuck method is that you don't have a lot of tied up in your rolling stock. Most of the wheels I've bought are around 100 -200 dollars a set. You an always upgrade when you find something you like better and there is always a buyer willing to take them off your hands.