I am leaning towards Bushmaster but the price on the DPMS seems right as compared to Bushmaster catalogue price ($259). Where should I be looking?

Get a stag lower from the equipment exchange, you should be able to find some good deals there.

Is there any quality/function difference in different brand lowers, or is the difference just the stamp?

I know there are forged lowers and cast lowers,....but other than that whats the difference? Internals?

Also....which brands are know for a good tight fit to LMT uppers?

How's their reputation? While the price is right, I know nothing about them and all I have found on their website are rifles with the controls on the wrong side!

Any lower from any major company will be fine, as long as it's forged. With the exception of a few companies who make the walls a bit thicker, all lowers are CNC machined using pretty much the exact same blueprints (you can actually find them floating around the internet)...hence the term "mil-spec" or military specifications.
The biggest difference other than wall thickness is the coating that is put on. I use an Olympic Arms T3 MaxHard receiver and that is some tough stuff, but you pay for it. RRA supposedly has a really nice finish, I'll find out probably in a week or two when my upper (eventually) comes in.
Cast vs. Forged. Cast means just that, there is a mold, there is some really hot metal, the two meet, and make a part. Forged means that the part generally started out the same way as the cast and was given a general shape. Then it was put on a CNC machine and finished off. Simply put, avoid cast, go with forged.

For you lower though, just buy a stripped lower, a kit, and a stock kit (don't forget the buffer, buffer spring, and tube). Go down to Sears, pick up some roll pin punches, a rubber mallet, and a hammer (if you already don't have them). Then go online and buy a stock wrench, Fulton Armory's "Complete Assembly Guide", and a detent installation tool (believe me, it makes your life much easier when it comes to putting in that damn pin). Sit down one night, and put it together. You'll save a few bucks versus the completed upper ($100 for the lower, $60 for the parts kit, and you probably have most of the tools) and you'll know exactly how your rifle works.

Quoted: Any lower from any major company will be fine, as long as it's forged. With the exception of a few companies who make the walls a bit thicker, all lowers are CNC machined using pretty much the exact same blueprints (you can actually find them floating around the internet)...hence the term "mil-spec" or military specifications. The biggest difference other than wall thickness is the coating that is put on. I use an Olympic Arms T3 MaxHard receiver and that is some tough stuff, but you pay for it. RRA supposedly has a really nice finish, I'll find out probably in a week or two when my upper (eventually) comes in. Cast vs. Forged. Cast means just that, there is a mold, there is some really hot metal, the two meet, and make a part. Forged means that the part generally started out the same way as the cast and was given a general shape. Then it was put on a CNC machine and finished off. Simply put, avoid cast, go with forged. For you lower though, just buy a stripped lower, a kit, and a stock kit (don't forget the buffer, buffer spring, and tube). Go down to Sears, pick up some roll pin punches, a rubber mallet, and a hammer (if you already don't have them). Then go online and buy a stock wrench, Fulton Armory's "Complete Assembly Guide", and a detent installation tool (believe me, it makes your life much easier when it comes to putting in that damn pin). Sit down one night, and put it together. You'll save a few bucks versus the completed upper ($100 for the lower, $60 for the parts kit, and you probably have most of the tools) and you'll know exactly how your rifle works.

Many, many thanks for your advice. Sounds like a good plan.

If you go the STAG route from Eaglefirearms, they don't charge any extra to put the lower together. So a complete lower half, is the same price as buying a stripped lower + lower parts kit + stock.  If I remember correctly Stag is the "house" brand of the company that make Rock River lowers, but I could be wrong.

FB

Quoted: With the exception of a few companies who make the walls a bit thicker

who makes the walls thicker and is there a fitting problem with different brand uppers?

Quoted: who makes the walls thicker and is there a fitting problem with different brand uppers?

RRA lowers have different dimensions and do not mate well with non-RRA uppers.