this is really a difficult question to answer.
My next tank will probably be designed and built by myself as much as possible - mainly because it will be pretty big.
Having said that, if I wanted a smallish tank, and had £1500 lying around, I would seriously consider getting a deltec setup, as they are really good quality and look good as well.
As for what you should do, it really depends on your experience, and how confident you would be DIYing.
Taking the 3 theoretical situations you mentioned...
1 - complete setup - ie percula 120 (bow front is nice btw)
pros - comes with just about everything you need, look good
con - expensive
this is a good idea if you are setting up a fairly standard reef system.
if you are keeping anything that requires a slightly more specialised environment, you may end up changing out a lot of the equipment, or adding a lot more.
if you are keeping fish only, there may be some things included you don't really need.
2 - custom setup
pros - obviously more options than above
cons - more expensive than DIY
Beware the lfs including some sub-par equipment with this sort of deal.
3 - DIY
pros - cheapest option, you know what you are getting, completely customisable
cons - need more knowledge of what you want, sometimes doesn't look as good as a complete setup.
Although I have bought quite a lot of items from Ebay, you have to be very careful about 2nd hand aquarium equipment - you don't know what chemicals have previously been used, parts will wear out quicker etc.
I think this is the best, and most satisfying option if you have the necessary knowledge and practical skills to put it together.
You don't say whether the £1500 includes things like livestock and liverock etc. which will of course make a difference to what exactly you can get.
