Why don't you stop by the public library and lbdk at a book on gunsmithing?
You need some way to hold the action without marring it, some way to remove the old bbl and some way to get the other one installed, then headspace.
There are all kinds of tricks to this, heating, restraining the action, various methods of getting a tool on the barrel, etc.
Of late I'm of the opinion that if you don't mind a silver gun stainless is the way to go. No finishing required - just install, adjust and go.
If you build a rifle, start with a geunine rem 700 action. Then go look around for the desired floorplate, which will include the trigger guard, magazine etc. You can stick with Remington's (decent) trigger or upgrade. Then select a barrel. You're not 'set into' a caliber until you get the bbl, and even then you can jsut get a blank and have it reamed to your desired dimensions. For your first gun, I'd start with a chambered bbl that just needed installation and headspacing on your action.
Note that you -could- also start with a pawn-shop beater-gun. If it was stainless already, or if you were going to finish it that'd be GREAT. So what if the stock is a POS? You were going to replace it anyhow. Just be prepared for some man-handling to get the old bbl off. But then you'd have your action, and it'd probably be nice and broken in. If you could get the gun for $200 you'd save about $300, because that's what new actions cost (or more).
The other thing you could do is go the M98 (Mauser) route. Get a new action or new barreled action from Charles Daly or the like. Personally I'd rather see you with a Rem700 or at least a QUALITY M98 (yugo's are decent). BBLs are easilly available to fit the mauser, but bear in mind your action length is going to be set at 'medium' for 308 and similar cartridges unless you spec. buy a long version (which is usually expensive).
1917 Lee Enfield would be another possibility, but I don't really like this action. Others do, however.
The Japanese Arisaka is considered the strongest action in the world, just in case you were ever wondering.