I have quite a few suppressors, so i know a bit about the process. the tax stamp is $200 that you pay to the BATFE, that is on top of the price of the suppressor.
for the 223 and 6mm, I would get one suppressor in .30 cal. I just got this one http://www.gem-tech.com/STORMFRONT.html for my 6.5x47. You will not be able to tell a difference in sound suppression between a 223 can and a .30 can on a 223. Some people claim you can, but if that's the case, it is a very very small difference. I cannot tell any difference.
I have a lot of different brands. AAC, gemtech, SWR, YHM, etc... All the big names make good products. But, for my recent purchase posted above was bought because it was Ti.
One thing about cans is that they can get pretty heavy hanging off the front of the gun, and you'll really appreciate the lighter weight. The Gemtech, sandstorm in .30 cal, is close to the perfect bolt gun can in my opinion. It weighs a 1/3rd of what steel cans weigh, but still does just fine in the suppression area.
there is a ton of info out there, so good luck, and have fun.
also, the paperwork process is not really that bad. usually 60-90 days. it goes by fast.
second also. if you make a trust to file the form 4 in you don't have to get finger printed, or get the sheriff to sign off, or enclose passport photos. i did two with the sign off, and all the rest i've done with a trust. the trust is the only way to go.
go over to silencertalk.com for a lot of info.