A couple questions

I am going to go AG as soon as I can get all of the equipment. I will be building a single tier system and the sites with burners are real vague as to the exact specs. ie. High pressure and how many BTU's. I am looking at this one http://prostores1.megawebservers.com/krazykajuncookware_com/Detail.bok?no=269
Or what about this one?
http://prostores1.megawebservers.com/krazykajuncookware_com/Detail.bok?no=121 It shows the BTU's and I assume it is high pressure with the high BTU rating. Am I wrong?

Should I get 3 piece ball valves or 2 piece? 3/8" or 1/2"?

I guess it all depends. I think the multi-jet burners are more intended for natural gas. The nat. gas burners are 100,000 btu's ... that probably should be good for at least 10 gal. batches

I can't be much more help to you, but I can offer this:

23 tip jet burner for $16.50 ... half the price of the one you listed.

I did find this one but will it work for all three burners or is it too much for the MLT?
http://www.bayouclassicdepot.com/bg14_cast_iron_burner.htm

I'm not yet into AG or even full boils myself, but I was listening to one of the archived shows over at www.thebrewingnetwork.com and they were talking about burners for use with more advanced systems. I think the episode was the one from the Sunday Session on 8-19-07. Anyway, I think the take home message for you was that the multijet burners from your first link are for low pressure systems and are not intended for use with high pressure propane tanks.

EDIT - Chris Graham from More Beer talks about them in the above episode. It's around half-way through the show. I couldn't find the associated presentation slides tho.

They make those multijets in both propane and nat gas and it's all about how large the orifices are. I'm not suggesting you use them but they do exist.