I guess I thought you had the basic background down already: Importing bikes to Thailand is a problem, either whole, or in parts.
Look here:
http://www.gt-rider.com/ There are many threads about bringing bikes in, registering them, etc.
Basically, if it's not made in Thailand, it's marked up over 100% (for whole bikes), and subject to emissions testing (at your expense) of the same kind as a manufacturer must go through to qualify their bike for sale in Thailand. That means about US$1000, anywhere from 10 days to six months with your bike at the testing facility in Samut Prakan, and no guarantee it will pass. No refunds, either.
If the bike is fairly new, it may pass, and you'll be eligible for a green book (registration document), and a number plate. If it doesn't pass, you're out the time and money.
The 100%+ markup can be bypassed by bringing it in in parts. Then it is ~30%, but based on valuation stated by the seller. You need an invoice, implying that you bought the pieces from a legitimate dealer. Japanese bikes are often brought in this way. When it arrives, the shop re-assembles it (correctly, if you're lucky), and sells it WITH INVOICE.
Read the bike classifieds on Craigslist, Baht&Sold, and Thaivisa. Most bikes over 250cc will say they're sold "with invoice, no green book", if they are up front about it.
Of course there are shops that specialize in importing bikes, and they MAY have invested the expense to qualify a particular brand/model, but they usually sell big bikes with invoice, and offer a green book as an extra-cost option. Some of these green books are EXTREMELY dodgy, and may not even list the correct displacement of the bike they represent. Usually these are forged, or tampered with, and the last thing you want, if you have contact with the authorities, is fake papers. Not to mention insurance, which will use ANY excuse to prevent paying a claim. One look at a dodgy book, and you'll be lucky if they merely walk away...by extra-cost, I mean in the neighborhood of THB75,000, or US$2500. Nice, huh?
Currently, I only know of two 250cc dual-sport bikes sold new legally in Thailand: the Kawasaki DTracker/KLX250, and the Tiger/Sachs x-Road 250, which is supposedly available now. Neither has the power to lift the front wheel, by throttle only, in second gear or higher.
I know all this, because I have the same desire for a lightweight, powerful dual-sport as you do. I tend to favor lighter bikes, and decided to stay below the 250 line, because there is about a 25kg penalty going from my DR200 to a KLX250. If I was going to cross that line, a DRZ400 or a DR650 would probably be my choice.
I had a Yamaha TW200, invoice-only, with fake number plate. I was paranoid the whole time I rode it. I actually only got stopped once, and all they did was check to see if I had a driver's license. But I was sweating bullets...
Here, in Isaan, it's probably OK to ride without a plate, but keep the invoice with you, and pay the tea money if you get stopped. There are at least two problems with that: insurance, and, if a cop likes your bike, it may get 'impounded'. You may then see it being ridden by you-know-who. Lately, I have seen ads for insurance on invoice-only bikes, but I would check very carefully what is covered.
The solution: no idea for your needs, but I have made my peace - I will not wheelie into town.
Good luck.