My advice is go for a 20″ wheeled one. Lots of people on ‘BMX advice’ threads recommend cruisers, and while I’m sure they’re great, if you’re thinking of a BMX, then you might as well go for the full BMX experience. Me and a mate both got one last year at the age 40. We’re both 6’1 and went for 21″ top tube. You could go longer if you’re tall but they aren’t so easy to find. We both get on fine with 21″ – use them for pump/BMX tracks, concrete parks and Rush Indoor Skatepark. Also for just razzing round the garden with the kids on their balance bikes.
Have a look on Winstanleys, they always have good spec BMXs at discount prices. I bought mine there – full cromo frame/forks, 3 piece cranks, sealed bearings, double-walled rims etc for £230.