I've got a 16" RRA Varmint upper, and stinkypete is right...it's a boat anchor...I can't imagine what 8" more of barrel on that puppy would do in terms of weight.
I've spent a lot of time thinking about building an upper, or just rebarraling the RRA to a 24" match barrel, but not a bull barrel due to the weight.
I haven't decided what to do, but I was looking at all the same components that you were. If I'm not mistaken, M&A has 24" wilson match grade barrels for $125 or so...you could slap that on the upper of your choice, along with whatever you want in terms of handguards, and it would still probably be cheaper than a RRA Varmint upper. I think I paid $460 for mine new, and that was last year. An M&A varmint upper is about $435.
[url]http://www.m-aparts.com/[/url]
Also, I just built a model 1 kit. The barrel seems fine, and out of the box with my 55gr handloads, the 16" barrel did 1moa. The only part I was unhappy with was the charging handle, it felt like it was made out of pot metal, and it was horribly rough feeling. I swapped it for a spare I had, no problem.
Stinkypete is also right about bullet selection; my 16" RRA has only cracked 1moa once or twice, but I've shot nothing but 55gr bullets out of it. I'll have to try some heavier bullets, since it's got a 1:8 twist.
I'm not a varmint hunter, but I gotta believe there's the need to handle bigger bullets for stuff like coyote, and smaller, faster bullets for groundhogs. A 1:8 or 1:12 twist might limit you as to what you can shoot; 1:9 or 1:10 may be a compromise. Serious varminters chime in here lol...I just shoot paper!
You could build an upper with a Kreiger barrel for a little over $600:
barrel $400
receiver $80
bolt/carrier $105
gas block $30
gas tube $8
If you're even considering the Armalite @ $600, I'd seriously think about the Kreiger, you could get any length and twist you want. Switch to a Douglas barrel and drop that price down about $150.
and, last but not least, Fulton Armory will do you a hot shit varmint upper for around $650, with a RAS style front end.
Good luck on making a choice lol...I got a headache and went to bed!