I think you need to start with why you want the Thumper.
The original Brit bikes all the way through to the last Japanese examples were singles for manufacturing cheapness and ease of owner maintenance. The Enfields seem to be treated like mechano meeting one idea, any 250-600 cc Japanese twin can be bought relatively cheaply and needs next to nothing doing to it. I have to say I've been looking at Honda NX700's with what is basically half a civic engine, it ticks a lot of boxes. The extra cylinder is actually cheaper than them passing on the development costs of FI and emissions package for a thumper.
If it's for the feel of how it rides, I'd suggest getting a go on the likes of a Triumph Scrambler or Kawasaki W800. Open riding position, narrow bike, just enough power but enough vibration to stop you getting bored, firing order and FI set up to feel older . It does feel right nailing these down the B roads with an open face lid etc.
I dislike the "adventure" tag, too many R1250GS's (Tourers) with Muriels of Ewan McGregor and pantechnican width TT boxes containing nothing but a Starbuck loyalty card, but you have to say that the smaller ones (General Purpose bikes) do what's required. The tank on the Wee is good for 300 miles, it'll do anything from gravel tracks to the autobahn and you do nothing but put petrol in it and drop the oil once a year. I have scrape marks on the pegs and side stand that in my mind show that with modern tyres the shape of the bike doesn't really matter. I think you could do the same with a G650 or Tenere or CB500X (I know, it's a twin).
I have decided to quit the old bike game. Trips on the RAC wagon add nothing to the experience and the lunacy of strident horns and indicators flashing at the wrong speed and bulbs that myseriously blow between dropping the bike off and the test that is the MOT is the rip off it feels like, a game I will no longer play. With this in mind I've started keeping a score of possible Wee replacements for 2015. Lighter 250 trail bikes lack range and have higher service needs. I see no need to go above 80 HP. The choices do indeed become limited and I will need to get a go on a Honda 700 and 500 at some point, but for me there are bikes out there that do what the Thumpers used to. The so called Glee (Gladius Engined Wee) is still favourite.
Edit to add: the Dealer lent me a Gladius when they dropped the oil at 600 miles. It was insane, breaking every speed limit felt like you were standing still, wheelies and getting your boot toe on the tarmac stupidly easy. It was like riding a video game and I'd be banned in a month if I got one.
There are also days when I do fancy another Enfield and the newer ones are pushing power ratings that may be practical. I lived with 18HP so I wonder if 28 will do most things?
Andy