Would be buying a car in the $2500-3500 price range, what are common problems with these cars, what can be done to improve reliability, how hard is it to get parts for, and how can performance be improved.

i dont have to deal with smog or inspection here so most of the emissions equipment would go to the scrap metal pile

 

Your up there, so many things are different.

Common problems:

Read ALL the sticky things on this site. Download and read Kirby Palms FREE "Experience in a Book", and read all 800+ pages TWICE.

Reliability. YES, they are super reliable. Trouble is ALL the so called EXPERTS that have messed with it for 20+ years. Now ALL this will need sorting, and obce that is done, reliabillity is your best mate. This is time consuming, frustrating, language expanding, and YES, you will need to get your hands DIRTY. If dirty hands is not on your agenda, then I suggest some serious rethinking, coz any mechanic you employ will only add to the mess already in there. Usually due to $$.

Dont worry about performance upgrades until you get the beast sorted as built by the "masters", then if that V12 is not quick enough, then do the mods.

When sorting whatever car you select, "short cuts" are forbidden. They will bite you sooooo hard and at the wrong time.

 

read the sticky and the buying guide, most likely i would be swapping in a 5.3 LM7 and 4L60E later on

i do not mind doing work myself and would be doing it all myself

 

if the original engine or wiring needed too much work, suppose i get rid of most of the wiring and engine to begin with and swap in a 5.3 LM7 LS and most likely a 4L60E theres no lack of XJ-S SBC/LS swaps so it is certainly possible and there are even kits to do so that dont require you to fab anything, but i am concerened about the wiring as i would like to leave a lot of the original harness in so i would be able to use some of the original guages and anything that is conmputer controlled except the engine, which would be controlled by the vortec ecu


what all is controlled by the original ECU such as dimmers, guages, climate control, etc?


does the original ECU need a signal to function and what functions with power on but the engine off?

 

Some of your questions would be more easily answered by knowing a specific model year. Broadly speaking, though, with anything 1991-ish and older the electrical architecture has very little integration. BCMs are not used; Gauges and body electricals are not ECU and/or BCM controlled.

Some of the subsystems (climate control, for example) have their own ECU but they don't integrate with the other ECUs.

Prior to 1987-ish there was only a single ECU for the fuel injection and it doesn't talk to anything else in the car.

You can click here for some excellent wiring schematics that will show how the electric systems gradually developed and became integrated over the course of years:

Jaguar electrical reference

Cheers
DD