BUY LOCAL!
there is no other answer.
not only is it more expensive to buy them online after shipping, but there is a good chance you will lose some in the move. this is a factor any time you have to ship fish.
(also, PERSONALLY, i would never buy them from anyplace that lists them under "tetras" while red hooks are more appropriately placed in "misc". it leads me to believe that they either dont have or dont share the proper information about this species...but thats my personal feeling/ experience.)
exodons are exported only during a VERY short period of the year. this is one of, if not THE determining factor in both price and availability. (and why they are not very common in the hobby). due to this, most LFS will not carry them, even if they show up on their order list. HOWEVER, if you ask around, im sure one of your LFS will be able to keep an eye out for you, or call their supplier to check on availibility.
buy having your LFS order them for you, they will incur the shipping charge, and take the loss if any fish are DOA. (thus saving you any further time and cost from having to ship you a replacement for the ones that didnt make it.)
it will also help support your LOCAL suppliers. this is a BIG deal.
create a rapport with your LFS. it will be both to your benefit, as well as theirs.
in regards to "how many". given your suggested set up. NO LESS then 10. and no more then 20. HOWEVER... if you are willing to provide better filtration and frequent w/c, you can pack more in there. (this max suggestion is based on your current filtration set up, and not knowing your level of knowledge/experience with this species)
as far as decor. thats completely up to you. its really a personal preferance. unless you are going to do a proper biotope (which would ultimately be "the best" for them) the decor is purely for your visual enjoyment. while adding plants/driftwood/ect, can provide some cover as far as direct line of sight, they will behave basically the same in both a planted and bare bottom tank. so it really comes down to what you want to look at.
hope this helps.