Tell tales I'd be looking for through an external inspection first is the condition of the coating (paint), and whether the casing itself is corroded, especially around the bearing carrier (surrounding the front of the prop). A poor coating, signs of corrosion, and spent anodes are usually a pretty good sign that the owner isn't that big on preventive maintenance. This has to lead one to wonder about the condition of hardware that you can't see. Also look for signs of impact. BTW, the Gen II has an anode behind the prop which sometimes doesn't get changed (lazy) as the prop has to be removed to do it.
The next easiest items to check are the bellows but you can't tell with the drive on whether they're leaking or not, you could though look for signs of aging usually presented by cracking in the valleys of bellows. To know for sure if the u-joint bellows is leaking or not you have to remove the drive. A bit of a pain when just shopping for a boat but with the drive off you can check other items like the u-joints, gimbal bearing, whether the input seal is leaking, etc. Repairs here can add up to big bucks if you're paying someone to do it.
You can shift the drive in and out of gear but the true test is of how that system is working is with the engine running and in the water. While there are adjustments that can be made, after initial setup, it's pretty common for shifting issues to arise from a bad lower shift cable.
While at the boat, if the battery is hooked up, check the operation of the trim limit circuit and the trim gauge circuit. Also inspect the wiring to the senders on the transom assembly's bell housing. The wiring needs to be in good condition with no bare wires showing or butt connectors. Butt connectors here are a bad shortcut.
I am not a fan of fins or anything else bolted to a drive that requires drilling holes, that's not a good idea. It compromises structural integrity, and it promotes corrosion.
Bravo drives are better IMO but they're not for every application. A couple of benefits are: water pump is on the engine instead of inside the drive as in the Alpha, and shifting is accomplished with a cone clutch which is a smoother shift over a dog clutch found in the Alpha. The cone clutch system also does not need to cut ignition briefly (interrupt circuit) to shift out of gear.
Not much you can check if the oil has been changed short of tearing the drive down and actually inspecting components (gears, bearings, etc.) You could use your nose to check for lingering burnt gear lube odours but fresh gear lube stinks to begin with. Check the condition of the gear lube in the gear lube monitor (bottle on the engine)
Parts are readily available for Gen II's and Bravos both OEM and aftermarket, you just have to be careful which parts you go aftermarket on. Can't say I like drives, they're just too maintenance heavy, and if you try and save money by skipping regular maintenance, you WILL pay big eventually. They help keep a roof over my head though
......I did like the pump access though.