I find myself in a similar situation. I purchased a secondhand fridge from Gumtree for $200. While it initially worked, it was plagued with various issues.
The unit has two small fans, but the rubber straps meant to keep them suspended snap, causing the fans to stall and rub against the polystyrene, triggering an audible alarm. Though I managed to fix the fans temporarily, they eventually seized up again, leading to more alarms. Subsequently, the cooling element brackets in the freezer broke, prompting me to strip out all the electronics and bypass them. The hours spent trying to fix this fridge have been considerable.
Now, I've reached a point where I'm contemplating giving up and investing in a new fridge.
I previously had a kegerator from Keg King, but its performance left much to be desired. The build quality was subpar, and everything felt cheap. I concur that modifying a good-quality fridge, ideally with the freezer at the bottom for accessibility, seems to be the optimal solution.
However, the challenge with secondhand fridges lies in uncertainty regarding their capacity to fit 3 or 4 19-liter kegs. It's always a hassle to request accurate measurements from the seller. Additionally, none of the manuals provide information on the location of cooling lines, making it risky to drill holes in the fridge.
Perhaps as home brewers, we should consider creating a database of suitable fridges, providing feedback on their suitability and performance.
Cheers,
Luke