I'm not a home brew expert by any stretch of the imagination.......but a kegerator, I've got that down pretty good. To start with, you aren't likely to find a mini fridge that will be easily convertible into a keg fridge. They have two problems, first the compressor hump in the bottom, and also the "freezer" up top. The freezer area is actually the coils that cool the entire thing. Find for a garage fridge for cans, but not much else. There just isn't enough space in these things to do much with them.
The most inexpensive route you can go is a standard fridge conversion. I've been using the same gear since 2000 (purchased used off of ebay then). It has been moved around from fridge to fridge on several occasional, actually had it installed in a university owned fridge when in college. With this setup, you have plenty of space for a full size 1/2 barrel, or a huge variety of smaller commercial or corney kegs. Conversion kits will vary from $130-$200. A standard top/bottom kitchen style fridge freezer combination can be had from Craigslist for under 150 bucks in most cases, you just have to build a platform to support the keg, the factory shelf won't be up to the task.
The one thing I will recommended before considering any conversion kit purchase is to figure out your local CO2 situation before you buy. You are going to have to get your bottle filled at a welding supply shop or possibly a fire extinguisher or paint ball supply store. Shops that will actually fill a bottle can be scarce at times, those that do fills on site have a tendency to make you wait to get the fill done. What I did was to pick up a 10 lb tank from my local welding supply shop, cost was $50.00 or so if I remember correctly. When time for a new tank comes, just give them a few bucks and they swap it out, no waiting. If it is going to be pain for fills, no need to pay for the 5lb bottle included with many kits. Besides, going with a bigger tank isn't all that expensive, and you don't have to get it filled nearly as often.
On my standard fridge conversion, I put the tap (going to be 2 taps in the next few days) through the sidewall rather than the door. This keeps from having any lines moving around when you open or close the door, and also retains full use of the fridge door for storage. Once my 2nd faucet/shank comes in, I will be set up to have two beers on tap, plus 3 more under CO2 pressure for carbonation. The freezer is nice for keeping frosted Pints/Mugs, ice, ect. Once note I will add here, if you are using corneys, keeping uniform post types will allow you to switch which beer is being served in seconds, you have to burn a couple ounces of beer remaining in the line. I am actually planning on putting a ball lock post coming off of my commercial coupler so that I can put it in and out of service as needed. Home brew is great, but personally I love keeping the option open to have a light weight domestic on tap for parties. Regardless, I would at least set up for 2 faucets. I don't mind anyone drinking my good beer, but if it isn't their thing and they would rather drink the cheap stuff, fine by me.
Below is a picture of my setup, I have about $350.00 total in it, including the fridge. You can easily go with a more attractive fridge if being used inside the house. For garage duty, this serves quite well. Another easy option is the freezer conversions, basically just have to add a tower or collar to a standard fridge conversion kit, plus a conversion thermostat.
Oh, most important thing to know about having a kegerator in the garage. If your neighbor knows the code to the garage door keypad, they are going to invade your beer supply, and probably be playing darts in your garage when you get home from work......most days of the week.