Xl125's used to have a bit of a reputation for throwing valves... most have the decompression valve disconnected ( small lever on the rocker cover).
If it has a current V5 and/or a reg number belonging to this vehicle that is registered on the DVLA database( you can get a V5 logbook for £25- if no problems with the vehicle with DVLA.)- then you should be able to mot n tax it.
If you intend to buy a motorcycle for road use but it has no regplate or V5 then youll have to go through a load of DVLA paper worth to have it re-registered, as there is NO way DVLA can find the original assigned reg on thier data base.
To be on the road it must have working lights ( indicators not needed-just remove indicator switch lever, but if theyre on they must work), display a reg plate as per reg's, have road legal tyres and an exhuast system that isnt too loud( mot'ers descression)
I dont know value or price, but take it for a test ride as i think theyre quite close ratio boxes which might be an irritant on the road.... as theyre were designed for off-road in mind.