The '88 will probably only be useful as a parts vehicle if it has the same engine and transmission as the '92, or if you need something like a door or tailgate. Ford was frequently updating and changing things on the Ranger during this time. Lights, trim, interior, etc. will all be different between 1988 and 1992. 1989 was a major update, as was 1993. Therangerstation.com is probably the most comprehensive website for older Rangers, but is geared more towards the off road crowd (though there are tons of resources there for all old Rangers).
Most I4s after 1987 have the Mazda M5OD manual, but the Mitsubishi and Toyo Kogyo manuals were used in some applications up to 1991/2. The transmissions are pretty easy to distinguish visually. Mitsubishi transmissions have an oil pan, Mazda ones don't. Most importantly, the Mitsu/TK transmissions used gear oil, and the Mazda used ATF (or a manual transmission specific fluid if you choose). I use GM Synchromesh in mine. The Mazda transmissions are known to leak from the rubber plugs behind the shifter, so check for signs of leaks near the top of the transmission. They also leak from the inner shifter boot plate that caps off the bushings. Dorman has a kit to replace the shifter bushings that includes a new boot/plate for around $20. M5ODs last a long time if the fluid level and condition are maintained.
The 2.3L went to the dual spark plug head sometime in the early 1990s, but I don't know what year specifically. Both versions are reliable engines, but the one with just four plugs is obviously half (or less) the work and half the parts to do a tune up on. The dual spark plug models had a slight HP/TQ advantage, but it was mostly done for emissions.
Generally speaking, most care and maintenance on these trucks is very easy. I can change the fluid in both diffs, the t-case, and trans in my 1994 in under an hour, and the most time consuming part is pumping in new fluid. There are lots of easy little maintenance things you can do under the hood like cleaning the IAC and throttle body.
Make sure the air intake duct doesn't have any holes and the rubber rings around the clamps are in usable shape. Also make sure the air filter box seals up and both of the push in nuts on the bottom half are still there. The air filter setup in these trucks is actually very simple and easy to clean/service, but simply due to their age and the number of hands that have been on them at quick lubes and the like, many are mangled up or not sealed well.
Check the suspension out to make sure everything is right with it. Look for cracks in the radius arm bushings (on frame below doors). Check the tires for unusual wear. If there's lots of negative camber it will eat tires quickly. A well sorted TIB suspension is tough as nails and actually drives pretty nice. One that is neglected or damaged might have a serious tire appetite. The steering linkage is all greaseable through 1997.
There's really not much to these trucks. They just use the cheapest normal fluids you can get at any parts store or Wal-Mart, and a fluid extractor will make any fluid change a piece of cake. Change the fluids, make sure the a/f and its duct work are good, check the brakes and wheel bearings, check and replace the hoses if old/original, and go from there. Rockauto is not surprisingly a great place to get Ranger parts...have found $4 Motorcraft coolant temp sensors, $66 Motorcraft reman MAFs, $8 Prestolite plug wire sets, etc. LMC Truck has a pretty comprehensive restoration/reproduction catalog for the Ranger, and NPD carries some reproduction body panels, bumpers, etc.
IMO, 1989-1994 is sort of an ideal year range for these trucks. They had very modern engine management for the time (MAF sensor, EEC-IV on most models), but were still very simple and easy to work on. For example, the heater core can be replaced on a 1994 model in well under an hour with basic tools. On a 1995 model, the book time is around 8 hours and the known short cuts don't knock a whole lot off that. While my newer truck is my daily driver, it's not for reliability reasons, the newer truck is just more comfortable and doesn't have mud tires on it.
Good luck with it and have fun. As for oil and filter, a '92 Ranger is about as non-picky as it gets.