1993 Mitsubishi Mighty Max Feels like its struggling

I just bought this piece of junk and really wish I didn't! Just to let you know, I used to be a mechanic about a hundred years ago so I know a little bit but the decision to buy this junker is really a blow to my pride. It has many problems in which I can fix but there is one problem that is confusing and could be any number of sensors, wiring problems, etc. Just to start off with some details, this thing burns oil in a noxious cloud when starting it after it sits for a day or so and in small amounts while moving. It's also hemorrhaging oil from what appears to be either the oil sending unit or the front seal, not to mention other smaller leaks. Not sure where, yet. When I drive it, sometimes the engine light will turn on and it will lurch and feel like I've run out of fuel or its vapor locking. I push in the clutch and give it gas and it gets a little better but still lags. After sitting for a bit, the engine light will not be on and it drives fine for awhile or sometimes even a couple days. Since I was looking at the engine (beyond the filth and film of oil), I decided to see if the coolant temperature sensor was plugged in since my gauge doesn't work. It wasn't, so I plugged it in and it ran HORRIBLE! It backfired, stuttered and the engine only smoothed-out when I was idling or sometimes while I gave it gas in neutral. I checked the codes and it came back with 14, 15, 21, and 1. I know what the codes mean but it would cost over $500 for me to replace all that and I'm thinking it's probably one of those problems. I know an O2 sensor can get screwed-up from bad exhaust like coolant, oil, etc. And send incorrect info to the ecm, so I'm wondering if you have any advice on that and/or where I should begin. Thank you. Mike.