What do you want it for?
To 'Collect' in which case marks and numbers along with appearance become all important and you'd better read the 'Bible' by Skennerton
To shoot for fun in which case get the cheapest you can with the best barrel you can find, then once you have played with it you can decide if you want to upgrade.
To go 'pot hunting' in competitions. Now the fun begins. You can wander round trade shows putting bullets into barrels as a 'gauge' just like Mr Numpty at
https://www.youtube.com/watch?v=UsNCqqWgZZA
You can hope the very expensive (£800+ when I last looked) 'Still in the wrap XYZ contract' rifle will shoot straight. You can think that High price = good and wonder how some with cheap rifles that seem to have shot out barrels can still outshoot you.
There are no easy answers. Get one that's not too expensive from a dealer that others have recommended or from someone in your club that looks after his gear and do some reading from RELIABLE sources.
Either use good factory ammunition or reload consistently and learn the proper techniques of shooting. If you fancy some old books look for AGs Book of the Rifle by A.G. Banks or for an SMLE the booklet The Service Rifle and how to use it. I'm not sure if I'm allowed to mention it on this forum but have a look at milsurps.com and the section on the Lee Enfield there are some quite good people on it.