I have converted a couple of uprights and I'm in the process of converting a chest freezer.
Most uprights will hold 4 kegs if you remove the plastic door lining and replace it with some masonite. If you scour the secondhand/reconditioned places you might get lucky and find a Kelvinator Trimline or similar - these have a flat door lining with removable & adjustable shelves - I have one of these and depite being quite a compact fridge I can fit 2 kegs above the compressor, 2 on the floor in front, and still use a door shelf...
(This one was given to me because the freezer door was busted - nothing a bit of polysterene won't fix!)
MAH - my understanding on the 'warm freezer' issue is that the compressor labours at warmer temps, as the pressures are higher until it chills down to normal working range. Normally, it's only once in a blue moon that the freezer is turned on at room temp (ie after moving or defrosting). However, when you use an external thermostat, you allow it to warm up so that every time it switches on it is substantially warmer than it would like. I don't know how true this is, and I haven't been able to get any sense out of a refrigeration guy either. I picked up an old chest freezer on eBay for $70 - so even if I kill it it won't be a tragedy. I'll also try to make sure it doesn't cycle too often, by enclosing the thermostat bulb if necessary to protect it from short-term fluctuations.
An upright is much easier to convert, you can safely drill the door and the sides without fear of hitting refrigeration coils, and you don't need to worry about temperature control. They are also easier to load and unload. I wouldn't worry about losing the cold air, the thermal mass of 4 kegs of beer is huge compared to the air that is lost.
However for me a chest freezer is the way to go, I can have 6 taps (4 full size kegs plus 2 shorties over the compressor), and the whole thing can go on the trailer and be kept cold by just running the generator a couple of times a day. I have drilled the taps through the front (by first removing the plastic trim and scraping out the foam insulation to locate the coils) and by adding some decent toggle latches to the lid I can make it secure enough to withstand highway speeds. As others have said, the flat surface is a nice bonus too.