Cor ! Where to start

First of all welcome to the site.

I think you already know that we could do with seeing some pics but even then cars can look great in pics but not so good in the metal, Anyway, I think if it was a GLi originally it will have been a 1.6 rather than a 1.8, the GLis were just what they were badged as in the early days before GTIs but that's by the by now as the engine has been converted. I'll try and explain what some of the things are that have been done to it.
16V 2 litre lump - Probably a 1.8 16v from a mk2 that's been bored out to 2ltr, a popular conversion but I think the pistons need to be upgraded for this though but not 100% sure
rebuilt 16v gearbox - Makes sense if converting the engine to a 16v, again probably from a mk2
Drilled airbox - This just means that they've drilled holes, usually in the wing sideof the air box to give more air flow to the engine, some say this makes a difference, others are not convinced (myself included). Also gives a more throaty roar when accelerating
K&N Panel filter - A better filter than the original to increase air flow again and these are about £30 - £35
Ashley exhaust 16v conversion - Sorry dont know anything about it
Avo coilovers - Replacement suspension that you can adjust so make the car sit higher or lower as required as they are adjustable, some say that if you dont adjust regularly the bolts seize up anyway. These will give a harsh ride.
Standard 16v Gearbox, Short Shift kit ...... - Gearbox as above and the short shift will mean that the gear stick will not travel as far making it easier and quicker to change gear thus making acceleration slightly faster and easier
Rear disc conversion - Mk1 brakes were originally drums and not the best so makes sense to uprate these if doing an engine conversion
uprated master cylinder - Makes sense when uprating the brakes
22mm Servo - sorry, dont know anything about but I guess inconjunction with the brake upgrade would make sense
285mm front brakes - I think the mk2 G60 has this size brakes and means better stopping on the front too
TSW Imolas - Dont know much about these either
It sounds like a lot has been done to the car and what he says makes sense but has he got any proof of this being done, receipts etc or has he done this himself ? I'm guessing that he's probably got this up for about 3 - 4k and if done right then it's probably worth it especially if the bodywork is as tidy as you say. How much does he want for it ? Are you prepared to pay what he's asking for it ? I think if it was me I'd get an AA inspection done on the car or get someone who knows what the guys on about to go and have a look at it.
I'm not really technical myself and the answers I've given are what I have found over the years of using this site, if you do some posts in the garage section you will get more technical answers from people about the conversions to the engine, gearbox and brakes.
The upshot is that something is worth as much as someone is prepared to pay for it, I'd also speak to your local garage that you use and tell them the info and make sure that they have the skills to look after the car with the bits that have been done to it.
Here's a good website to look around to see if you can get any further information -->
http://www.matey-matey.com/ This is supposed to be a good one too -->
http://www.vwvortex.com/If anyone else reads this and would like to correct me on anything then please do.
