How much will that car be worth when nobody cares about a JDM car anymore?

Yeah, its cool and all, and if its something you REALLY want then go for it, but its seriously pissing money away. The car won't go up in value, and if you modify it any it will only go down in value. To keep its value you really need to keep it OEM as possible, or easy to put back to OEM. But this isn't a numbers matching muscle car or anything, there really isn't a market to sell it on except for a limited number of other people with more money than sense.