if you are going to use the 5 foot tank, make sure it hasn't had any copper based treatments used in it ie. any whitespot ones as copper is toxic to marine inverts. the easiest way is if you are the only owner of the tank of course, but if you aren't, check to see if the silicon is discoloured in any way.
As for equipment, don't skimp on the skimmer, if you want a reef tank you can use normal fluorescent tubes if you only want to keep soft corals, but lps and sps usually need more intense lighting.
if I were you, and the 5 foot tank is OK to use, I would use that, drill it and keep the equipment in the sump. keep an eye on the specialist marine forums in the UK for 2nd hand equipment bargains and also live rock from people breaking down their tanks as it's more likely to be mature enough to use in your tank straight away and a fraction of the price you would pay in a shop.
