I'll personally vouch for Nick at GT Garage for tuning, as in actual tuning/ecu programing. Kutzie at Pro Automotive has a really good workshop and is a good tuner. I also have had most my mechanical work done(when I've been overseas) by Marty at Mobile mechanics, top guy and knows modified imports very well.
As for your issues: Buying anything modified, especially when you arn't familier with the particular style of car is a risky buisness. Not to mention you did buy it with unknown problems.
Common skyline ignition faults/issues are well documented, do some research on this site to find the details. You'll just keep fouling plugs untill you fix it properly.
As for your timing issues.... if the cam gears (are they genuine hks) were as loose as you say and the belt was super loose, you may have knocked the valve or 12 against the pistons. If so, you'll have bent the valves and damaged the pistons... thus you'll need a new engine. A new timing belt isn't going to fix it.
What kind of ecu is installed in the car? Is it programable? If it's stock, you can't make changes to the cold start program anyway... How modified is the car? Unless it has some crazy cams of serious head work, the stock cold start map should be pretty good. If there's been no change of injectors, afm, turbo etc.. then the car shouldn't need a tune either.
Personally it sounds like you need to get back to basics with the car, test and adjust everything factory first to get it starting, idling and running correctly before you take it to a workshop for a tune.. In most cases, the workshop won't tune your car untill everything functions properly anyway.
Cheers
Justin
PS If I lived up north, I'd make the trip to see Brad Sherriff to get things sorted out.