I would steer away from Montesa 315 as they are a very good bike but spares are getting hard to find, I was going to get one so i rang a Honda dealer just to ask about spares, he told me that piston rings are no longer available for them, they have 3 rings and the top ring is a special one that was only made for that engine and is no longer available. He advised me to get a Beta,Sherco,or Gas Gas as parts for these bikes are still available.
I got a very good condition road reg 2007 Beta 250 Rev3 for £1500 its ideal for what I want.