well i have no real experience with building acrylic tanks, so i will not comment on that. but i will about the filtration questions.
it might be a little cheaper (and easier) if you buy a tank. the acrylic you will need should be around 1/2in to 3/4 thick, which for the dimensions you listed, will proably be more expensive than a new tank. usually searching your local newspaper's classified section for a used tank will get you a good deal. at most you might have to reseal it, or replace some plastic molding but usually the savings are worth it. lastly check this forum's for sale page, people often sell equipment, fish, etc there and any garage sales, sometimes people sell them there too.
with all filtration you should try to turn over the water in the tank at least 3-5times per hour. if you have a 100 gallon tank, plan on getting pumps/filters that run at 300 - 500 gph
to have a sump you need to have a few things, either an overflow box, or a drilled tank with an overflow system installed (if you choose to build the tank, this would probably be the best way to install an overflow) the overflow box drains water from the main tank, passes it through a prefilter (like filter floss) and then into one end of the sump, which is nothing more than a large container usually about 20-30% of the size of the main tank, the bigger the sump the better, but shoot for at least 20%. the sump can be anything that holds water, like a rubbermaide container, another tank etc. placing baffles into the sump to direct the water through the sump will make sure water spends more time mixing with the filters, and media. filters can be anything from hob
models, bags of media both carbon and biological, bio balls, etc.
Sumps will usually house heaters as well, and various other things if you do saltwater [like protein skimmers, calcium reactors, etc]. then at the other side of the sump, a pump returns the water back to the tank.
should tell pre built sump / wetdry units, that only need a pump, and to be plumbed into your system. they can become more expensive the bigger and more intricate they are. if you were to build anything the sump is what i would suggest.
Marine land, cpr, and aquarium systems (i think) are all manufactors of sumps/wet dry filters.
here's one, http://www.marineland.com/products/c...n_tidepool.asp
-the tidepool, side note: 139.99 at thatpetplace.com
but all sumps need an overflow, never use a 2 pump system (1 to send, and 1receive)