Have to agree that going second hand is good my first big tank was second hand and never had any problems you can usually find them in local papers or ebay the main advantage of buying from a shop I guess is that they will be slightly more willing to give advice and also have a better understanding of where you are in your fishkeeping (an other way of doing this is spending a hugely inappropriate amount of time in the shop and get onto first name terms with the staff
)
Other set up expenses for me after just setting up a much smaller tank were tap water conditioner which depends on how large a bottle you buy substrate which again varies on what type it is but I think that the standard gravel is about £9 for a large bag at the moment getting live plants for your tank would be advantageous think I said on your other post about giant valis very fast growing so would look good in a new tank and quickly fill out so perhaps realistically £20-£30 on plants and also lead weights. Also depending on what you like rocks or wood. If your going for wood again it varies from place to place sometimes your lucky and get pre soaked chunks for about £3 other times you find a root tangle you like for £15 + For rocks you can get quite a lot that are appropriate for tanks including some that look like mini mountains but these are very fashionable with hardcore aquascapers so they are quite expensive where as slate a good material IMO for larger tanks are easy to find at garden centers for as little as a pound a stone.
Also a good idea to stock up on replacement filter pads when for when you need them but that could wait a few weeks if you like.
Other tools that are useful if youve not thought of
Buckets - one normal household size and a large one for holding water for water changes
Syphon - for sucking out water and cleaning gravel
Measuring jug - if you ever need to do small changes
Cheap colander - for cleaning gravel
Thermometer for the tank - sometimes not included but needed - though often cheep
think thats all I can think of
edit - a multi plug - heater + filter +lights are usually 3-4 plugs
also good choice on tank size good to start out at the size as they are better to keep stable.