right just found this thread and you aint far from me so what gars you plannin on? i've never seen any that will survive the uk winters!
now for the tank build......
for the ply the cheepest i have found DELIVERED is arnold levers. you can go in to a warehouse and pay for it then they will deliver for you
if you have a van you could prob save a few quid buy trying some local dealers or building sites
accrilic is not cheep and DON'T try and skimp on it you want to go a bit over the top on the thickness i would say 20mm MINIMUM for the smaller sizes of tank you have been talkin about
normal sponge will be too dense use pot scrubbies they are even cheeper than sponges so you will get better filtration for less money
sturgen and channel cats will produce alot of waste so i would re-think useing a sump as a 10 x 3 x 3 will look small when they start growing a bit
it is easy(with a little elec knowlage) and pretty safe to run mains outside to run filter /airpumps etc as long as you use the correct exterior fittings and use a breaker, this would imo be much better than battery pumps
i have read up on building a ply tank and for my own peace of mind i will be fiberglassing all the joints to try and minimise leeks
if the tank is not up against any solid walls (house walls not shead) it will need alot of braceing! i dont think you understand the pressure that will be in it, it says your at collage on your profile so i'm assuming you are 18-20. so as a quick example, when you fin buliding your tank you should be able to stand in it and kick the sides as hard as you can with no effect.
i hope i dont sound like a d*** head when i post this as i do wanna help but for a 10 x 3 x 3 diy with diy filter i would guess you are prob still gonna spend £1000 so it aint something you can doo cheep and it wont be that easy, i do alot of joinery and have access to a very large joinery workshop and i am still wondering if i should attempt a big ply tank indoor as if anything went wrong and 300 gal ended up in my front room my life would not be worth living lol
hope this helps