Second that parts are hard to find in some cases, especially for things that commonly go out (turbo inlet pipe *cough cough*) However, in most cases an aftermarket part can be found or fabricated. The 11mm injection pump from the newer cars fits on the AHU with some modifications so I wouldn't worry too much about that. Seals can be replaced it is just leaking.
That said, I wouldn't rather have any other car. The gas mileage combined with engine power is phenomenal. You should get low 50s for hwy driving which can't be be beat. These cars CAN get up to 200 hp when heavily modded. Mine is around 120 hp. For a small lightweight car, that is PLENTY of power. However, the stock power is only 90, so you may want to plan some upgrades if that interests you. For about $1k you can get 30-40 more hp with some fairly simple mods.
EDIT: Only other major downside to these cars is they are not meant for offroad use without modifications. You will feel ALL the bumps. Clearance is very minimal. These characteristics can be slightly altered or improved upon but it's also FWD only, so keep that in mind.
About the car you are looking at specifically.... It looks scarily good for how many miles on it. The maintenance he put into it recently looks solid though and will save you some money so the higher price is most likely worth it. Most of the things that commonly go out are likely already taken care of at that mileage. Mine is 211k and I've done A LOT of work to the car. So I think the price is good especially considering new interior upholstry, new turbo, brakes, working heater and AC, etc...
ask about the clutch - might be new at that mileage also. Good to know if it's stock, or aftermarket. If you want to mod it you may have to get a more robust clutch anyway.
ask about the injectors (ever been replaced, if so what size nozzle?), or if you the owner ever got it chipped. what turbo is it?
ask about injection pump. seals replaced? new pump?
ask about timing belt - every 60k miles, needs water pump and tensioners at the same time
ask about the the turbo inlet pipe. again, at that mileage it must have already been replaced. This is a pretty common failure.
regarding the heater, see if the blend doors have ever been patched. I don't know how the heater would be working any other way.
ask about engine work - camshaft/lifters, head, see if a compression test has been done on it, or ask for one, will give you a good idea of the health of the engine. At that mileage it is worth checking. Usually nothing that can't be fixed, but it's good to know what you are paying for.
what is the red thing next to the coolant ball? oil catchan maybe? Definitely not stock whatever it is.
The coolant ball is the wrong color. These cars need the pink G12 collant, not sure whats going on with that.
check for codes with a VCDS
check for rust on the underbody, skid plate intact?