Well there's a good few to choose from at the minute,both on eBay and Facebook.
To be fair there's only two engines GM and JAWA if your looking for a laydown bike.
Ideally take somebody with you who knows a bit about the bikes,when buying.
Cookie.
You'll probably pay less for a laydown than an upright !
May as well start off on the modern stuff,then like you say when you get a bit more experience,you wouldn't have to change again.
Cookie.
Just type in Speedway or Grasstrack,and various pages will pop up,click on and have a look or leave a message saying what your looking for.No issues with using a long stroke engine,all depends on the level your riding at now,if just starting out to be fair won't matter what engine you have.Kevlars are the most popular suits at present but people do still wear leathers.
Cookie.
Now and again Alan Bellham sells the odd one,but really advertising here and on Facebook,is a good way,also eBay.Other that that try popping along to any Amateur Meetings near you ie STR,Dragons,Scunthorpe etc,also you find Jawa come up more often than GMs and are often cheaper.Hope this helps.
Cookie.