Just in case you have not ordered the filters yet, I thought you might like the idea of a Fluval FX5 for £137.50...
http/cgi.ebay.co.uk/Fluval-FX5-External-Filter-New-UK-Model-/310184913438?pt=UK_Pet_Supplies_Fish&hash=item4838762a1e
I have ordered one this last weekend after my own filtration dilemma for a 5x2x2, when I was considering two of the All Pond Solutions 2000EF for £150, but decided that FX5 price was too good to be missed for a filter with an excellent reputation. I will supplement it with a 2000EF in either October or November.
You will want some media for the FX5 or even an APS filter ("hamfist" and others have said they think the supplied APS media is not enough) and the best deal I've spotted so far for Eheim Substart Pro is just short of £50 on Ebay (yet to order myself, but on "to do" list)...
http/cgi.ebay.co.uk/EHEIM-SUBSTRAT-PRO-5litres-Filter-Media-Aquarium-/150485166773?pt=UK_Pet_Supplies_Fish&hash=item23099d6ab5
You might also be interested in the other deal I ordered over the weekend, a Hydor Magnum 8 powerhead (12500 litres per hour, my riverine fish will love it) for the superb price of £106.21...
http/cgi.ebay.co.uk/NEW-HYDOR-KORALIA-MAGNUM-8-CIRCULATION-PUMP-12500LPH-/150455452490?cmd=ViewItem&pt=UK_Pet_Supplies_Fish&hash=item2307d8034a
I believe Bosemani Rainbows, if still the centre of your plan (its great to see someone giving these wonderful fish a good sized tank, rather than "squashing" them into a 4-footer), will enjoy current. "drobbyb" is our resident Rainbow guru in my opinion (breeds many species in USA) and he can help you decide on this idea.
If you have not got heaters yet, the Rena Smartheater 300W are highly rated. I've ordered one for £22.49...
http/cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=310247495700&ssPageName=STRK:MEWNX:IT
You will need two of these. I'm combining a single one with a Juwel 250W for now (as I only need a temp of 22C for my setup), but I may well order another in the near future.
The Rena heaters can be connected upto many brands of extrernal filter piping, with a little kit. I ordered one for £6.49, but you may well find cheaper elsewhere...
http/cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=310198911715&ssPageName=STRK:MEWNX:IT
I believe its one kit per heater, so you may want two. This will pretty much eliminate any "dead spots" containing cooler water.
I have no suggestions for lighting! But if you come across any great deals for 150cm kits, of any sort (I only need basic lighting to support Anubias and Java Fern, the only plants that can survive in my setup), please let me know.