I think the best one to mod is the 1999.5 or 2000 golf GL 2 door hatchback. Second best is 1999.5 or 2000 golf gl or gls 4 door.
After that i'd go with a 2001 or 2002 Golf GL 2 door, or 4 door.
They were made in germany in 99.5 and 2000. The gl 2 doors had crank windows and NO extra stuff getting the curb weight down around 2600, with some padding removal I'm sure you could get below 2500.
Germany is a good thing, they have a much higer build quality than 1st or 2nd year brazilian or mexican production. third year maybe, fourth year they were at PD anyway. Friend has a 2000 golf 2.0. German car, mexican engine. Guess what, it has been a flawless car but the engine smokes, clatters, snorts and generally acts like it has asthma and a bad limp. 2000 tdi's still got the german engine in the german car.
All Golfs 2001 and newer are latin american origin. I think the wagons were made in germany, but those are not light vehicles, particularly when compared to a 2dr golf gl.
You have the lower weight and added stiffness of a 2 door, and all the GTI bits you can get your hands on will fit exactly. Us 4 door guys have to use a mixture of GLI and GTI bits due to the 4 door nature of the beast.
If I were a modder, and had to buy newer than 2000, I'd insist on a 2 door with manual windows, as south american VW's aren't known for their wiring IMHO.
After that you have a 2003 2 or 4 door golf, but power windows only. Finally in 2004 they start the PD, which is a powerful engine, but I don't think it will have the longevity of the original style pump, and there are a lot fewer mods available to you.
Hope that helps. All of the pre-PD mk4's are the same as far as what you can do to them, but you will have more fun tossing the 2 door golf around after it is full of go-go juice, sticky tires, and aircraft carrier cable brakes.
I went with the 4 door golf as it is far more practical, and I like the sunroof and power stuff because I am lazy american like the rest of us. That and it was a good deal when TDI's were beginning to skyrocket in price. If I were in a mode where it was a fun car and didn't need to be practical I would have held out for a two door most likely.
take care,
Jester