Firstly get the UK R2 it's much better than the fairly plain Jane R1.
Secondly I'd say yes (how much are you a fan?). Personally I loved DD and found both cuts add something in different ways. I'm keeping both my R1 TC and R2 DC. If you're lucky you'll still be able to source the holographic LE version.
As for the extra scenes according to the DVD Times review they greatly dumb down the film, personally I found the opposite and they create a different Donnie Darko but one also very good. I was still confused at the end which must mean something.
As for the extras roughly 1/3 are the same extras from the R2 TC DVD (including the deleted scenes some of which are in the DC) and the rest are new they're decent but not top quality. Don't compare them with the likes of Gladiator but they're better than tosh like HBO "let's gush all over the director" pap.
The LE looks to be available from the UK arm of CD WOW (cheaper than the US or HK) for £5.99:
http://www.cd-wow.com/detail_results_2. ... ffid=13858
It's the cheapest I can find it on the web.