Welcome to the forum.
Have a read of these:
http://www.airsoft-forums.co.uk/index.php/topic/3779-the-beginners-guide-to-airsoft/
http://www.airsoft-forums.co.uk/index.php/topic/17294-ians-guide-to-noobism-on-a-tight-budget/
It takes a lot of time, effort and money to set an electric airsoft gun up to be effectively be used as a DMR (or sharpshooter's rifle). In airsoft, these are essentially electric or gas guns which have been locked to semi automatic and have had a lot of internal upgrades put into them.
I would suggest buying a normal AEG first before going anywhere near DMRs. Anything in the G&G Combat Machine series of M4s and AKs is perfect for beginners. But even before buying your first gun, you should go to a skirmish and hire the kit out for a day to see if you actually like it first.
As you're over 18 you can get an airsoft gun which has 51% of it painted in a bright colour (such as orange, green or blue). These are classed as Imitation Firearms. The dark coloured guns (Realistic Imitation Firearms) such as black, olive drab, tan, camouflage etc require a UKARA defence, re-enactor's defence or a site membership (some retailers will accept site memberships as a defence) before you can buy one. It's illegal for a retailer or an individual to sell you an RIF unless you can provide a defence such as the ones listed above. This is just to prove that, in the eyes of the law, you are just an airsofter or a re-enactor.
In regards to ordering guns from abroad, Customs have a history of destroying packages with airsoft guns in them unless a UKARA number is written on the outside of the box. Some retailers in Poland will do this.