You can get a dish in B&Q, but personally I would steer clear of the bargain basement Ross set-top box they also sell. You should buy a Freesat-branded set top box (extensive advice and price comparisons are available at www.joinfreesat.co.uk
) and give yourself the option of using some of Freesat's extra features such as a full 7-day programme guide and the BBC iPlayer (also ITV player on some boxes).
As for installation, the most difficult part is aligning the dish which is far trickier than aligning a terrestrial aerial. Invest in an electronic satellite finder to use during your installation for minimum hassle.
Plenty of people have done DIY dish installations so there's no reason why you shouldn't be able to do it. Just make sure the dish is firmly fixed to the wall, the cables are neatly cut and the f-connectors are properly fitted with no stray braid hanging out of the cable and waterproofed where they connect to the LNB with self-amalgamating tape.