Ok. No experience on Impreza nor WRX.
Have you done at least Sing - Penang before?
IMHO, buying a 7-8 year old car for this trip may not be wise, especially a TC'd one. Why?
1. There may be issues with the car you may not know about. Turbo charger bearings may be going, pistons, valves, bearings, etc, etc. It is best to buy the car, and use it for 6 months, to see what issues it presents.
2. For a TC'd car: Just look at this forum. Many who sell will de-kit. Replacing parts which could be sold to recover costs. Some may have had tuning done, and may not have paid tuner to tune back parameters to accept the replaced parts. UNLESS, you buy a fully kitted car which had not de-kitting done.
3. Do you know the history of the TC'd car? With aggressive mods and tuning, you can expect high/heavy wearing and tearing of parts. Car may be sold just before wear/tear issues manifest itself, and big ticket repair/replacement costs. 7-8 year cars ought to have a total engine overhaul done. (A friend of mine took over a car, and the engine bearings failed, found pistons cracked, compression test failed. After that, it was one repair and replacement after another .......)
4. Condition of radiator?
Comfort? What is the condition of the seats? The cushioning still ok? Suspension ...... Is it too hard? too soft? Worn?
Tyres,...... 20% 30% left? What tyres are on?
Rims ...... Warped? Cracked? Bring good spare tyre, tyre repair kit, tyre sealer, tyre pump. Sometimes, you may not just get 1 puncture!
For me, I would not purchase a 2nd hand TC car. Period.
If you were to look at cars on NS highway, even the Kancil can also make the NS highway Tip to tip.
How fast or how hard do you want to push the car? For me, my last ride had uprated radiator, add-on oil coolers. Heat can be your greatest enemy.
There's so many things to consider.
Have done this run. It was fun! Had to bring bottles and bottles of Octane booster, tuned down ECU to accept 95 fuel.
Getting a good conditioned Vios can even do the run.
Apologies for the fragmented thoughts. I've not covered all I might want to say, but, tired liao.
You need to be totally familiar with the car (including all it's quirkiness).. My suggestion would be to take whatever car you are driving now, unless it's not in good condition and falling apart.
All the best for your trip........