There are 3 brands of thompson worth even looking at: TM and King Arms.make the best ones by a long way. Cyma do a basic one which should be okayish.
If you read the legal section, the rule about being 18 applies to the buyer in selling a RIF (non 2-tone)or IF (2-tone). It doesn't apply to converting an IF to a RIF. As long as you can prove that you have a valid reason (ie that you are an airsofter) then you can remove the paint.
L85's are cool but almost all versions have reliability issues so I would suggest that they are a good 2nd main weapon and not really good as a 1st buy. You want something reliable that will not break down as a fall back before you get something more exotic.
If you want something different to the usual M4 and you already have a pistol for any single shot only games, ASG (also sold as AGM) make a very good Sten. It has several drawbacks in that it has no semi auto or safety and you cannot get high cap magazines but it is relatively cheap, it's very reliable and accurate and its made of steel so will not break if you drop it. You will want a few spare magazines for it as they are not high cap.
If the person buying it has UKARA: http://www.taiwangun.com/en/electric-3/agm-058?from=listing&campaign-id=19
Otherwise: http://www.onlybbguns.co.uk/058-agm-mkii-british-ww2-sten-aeg-rifle-full-metal.html#.VuyS7fmLSUk
Incidentally I do not suggest buying stuff from onlybbguns.co.uk unless you know what you are looking for, like all sites with bbguns in the name they do sell some rubbish. They do however sell a few good items and I have never had problems with them.