Welcome aboard
The standard TDI (75) is very economical and reliable on the whole, although it can be a little pedestrian. The TDI90 has greater torque and driveability, at the expense of some economy, but its VNT turbo and DMF are costly weaknesses. A '75 running a mild remap is the best of both worlds for many.
You need to decide up front if you are interested in the panoramic glass sunroof (OSS), or climate control, and whether you want 4 seats or 5. These are the only things which cannot be retro-fitted so you need to look for cars that already have your choices fitted. Most cars above base spec had climate, but there are a number where the option was declined and they just have the standard heater controls (three rotary dials, and no AC). Note that the 5-seater rear bench only has ISOFIX support for one child seat, in the middle or nearside positions.
In terms of what models you'll find out there, there are mainly three - base, SE and Sport. Base models had 15" wheels as standard, which give the best ride compliance. SE models got 16" wheels, foglights, leather trimmings, a CD player (2002/51 onwards), and the false floor. Sport models appeared in 2002/02 and also had the leather trimmings and CD player, plus 17" wheels (fragile, least compliant ride) and lower suspension, a trip computer and Sport seats with deeper bolsters. Towards the end of production, there was a run-out Special Edition model which was base spec plus climate and a CD player. If you see anything out there wearing an S-line badge, someone's stuck it on there, there was no such model in this country.
You may be aware of the Colour Storm edition which was available on base or SE models (although you could still have it with 17" wheels and Sport suspension). This gave vibrant paint colours including Sprint Blue that you may be thinking of, with matt black roof, arches and inserts, and co-ordinating seat fabrics.
Sport seats had always been an option before 2002, but at that point lumbar support appeared and was fitted to them as standard (so it's in all Sport models). This is something which many find is lacking on the standard seats as fitted to base and SE models, but it only remained an option there.
Within your choices of seats, climate and OSS, for the long term you'd do best to buy on condition and history rather than mileage and options. As previously hinted, any of the above mentioned options and more (e.g. leather seats, BOSE sound system, cruise control etc.) can all be sourced and fitted to your car if you're keen enough.
There were some significant build changes during the production run. Cars from roughly 2002/52 have a larger 42l fuel tank, but around that time the original cast suspension arms with integrated balljoints were also replaced by pressed steel ones with replaceable balljoints. However these arms are prone to rotting out from the inside. A car where the arms of either type have already been replaced would be an advantage.
Hope some of this helps!
Best regards from another Ian .... there are quite a few of us on here