I suppose the answer to your question depends somewhat on what you're going to do with the bike and what you ultimately are after. For what it's worth, I'll tell you what I did with my 1990 KDX last winter.
I bought the bike new and raced it (hare scrambles & enduros) the first couple of years but took up racing stock cars after that so the bike sat. Well, now I'm back into the bike and decided to keep it and update it rather than spend $4K on a new KDX or who knows how much on an alternative. I've been doing a little racing and quite a bit of woods riding and the bike is still faster than I am at this point.
Last winter I stripped the bike down and sent a lot of pieces off to Fredette. I had the cylinder and head ported, the forks re-sprung and overhauled, the shock overhauled, and the top end replaced. I also bought an FMF pipe and silencer, new suspension and axle bearings, new water hoses, etc, etc. I found a lot of new plastic on ebay. In the end the bike is in like new condition and suits me quite well. Power is great but I am considering an upgrade to KX forks. I think I've spent somewhere between $1500-2000.
It seems worth it to me but you'll have to decide for yourself. If you're just after power, I'd suggest the exhaust and a good port job along with correcting the jetting.