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A Remington 700 action or complete rifle would be a good place to start, unless you have the funds for a better build with a true custom action.
You can part out the 700 stuff you don't need to help out with the money part.
The 700 will probably need to be "tweaked a little" so there is more money spent.
A custom action has that done already, and only needs a good barrel installed to start shooting.
The custom action will always be worth more money than a tweaked 700 will...
The necks sizes are just a control over the brass in a tight necked chamber, and now days most folks are going with a standard no turn neck when they spec out a reamer to use...
I do like to turn them a bit to get things all even but it's not really necessary ...
Lapua is as good as it gets as far as brass for any given caliber they make it in...
My newest build will be on a Lawton 7500 action with one of there barrels in 6mm, and Joel Russo will start on the stock work.
Lots of other places to buy stuff that you would prefer to use for YOUR rifle.....
Your probably not allowed as much fun as your headed into