How about a 2003 - 2007 DT125RE? it has a powervalve set up, de-restricting at the clocks gets you to around 80mph, nice riding position, you can play about with the suspension dog bones, and lower it, (longer dog bones shorten the ride height) electric start, and depending on budget anywhere from £1000 - £2000, will get you one.
Not a thumper like the XT, but for an all round fun machine, does what it says on the tin, till you get your DA, anyroad.
Just a thought
P.s I suffer from back problems too, and once I've lifted leg over the bike alls fine, plus I raised the bars on mine so it gives me a better ride height, (straighter back)