What you're proposing is fairly ambitious, but not impossible, it depends on your budget and facilities.
-I want it to have pretty decent range/power and accuracy
This means using a long and tight fitting barrel, at least 2 feet long.
-it needs to be portable i.e. have it's own air source (I realise each shot will get progressively weaker)
-I'd like to get like at least ten shots out of each refill of the reservoir/tank
-fairly cheap
You have several options here, but the "fairly cheap" clause is limiting. Ideally, you could fork out $89 and get something like
this, but that might be beyond your budget. The cheaper alternative is to have a large chamber at 150 psi or so, or a smaller chamber taken to high pressure with a shock pump.
-the simpler the better
The simplest valve option is using a blowgun directly as a valve, like so:
At pressures of 100 psi+, this will give good enough flow for
decent power through a long enough barrel - provided you squeeze it quickly enough. It will only open for brief instant, giving you much more than one shot per fill.
As to reloading there are several easy options, simplest of which is probably to cut a slot at the breech end of the barrel big enough to fit a projectile, then sleeve it with an appropriately sized piece of tubing that either rotates or slides back and forth.
