Up to you really but I wouldn't keep it stock just because you think it might become collectable and be worth more if its original. These cars aren't worth that much in comparison to other Japanese cars of a similar vintage like Mazdas and Datsuns etc. You just have to look at the prices a stock ke20 is worth to see potentially what your ke30 may be worth in a few more years. Also being a 4 door it is the least likely model to be sought after.
If you would be happy driving around in a stock ke30 then keep it stock. If that's too boring then modify.
To do it properly (ie engineered, brakes, suspension, diff etc) anything turbo will probably cost you 10k at the end of it. I have heard of people spending that on 4age's.
If your on a shoestring budget just get a ca18de and be done with it.
Seems to be a popular thought for people from Victoria. Seriously though why would you want to go to all the trouble/illegalities for a 3.8 VN motor?
Put a 1G-GTE into it. Its legal. It makes more power stock and it has potential to make a lot more. Plus its been done HEAPS.
Yeah its a very nice drive, whether your going the speed limit or maybe slightly above when there isn't a slow car in front. Its a relaxed atmosphere with lots of different people up there. Not sure what happens up there at night time. Have no problem believing it would get old fast living up there putting up with it all night.
Yeah R31 Silhouette are one option. Pretty sure there are some SAAB 15inch steel wheels that can be used with some modification.
If you want 14's R31/Pintara wheels are much easier to find and look alright.
It all depends how agressive you want to go. You will find people say a wide variety of wheel sizes "fit". However, everyones definition of fitting is different.