Buying and modifying vs custom build depends on budget issues and your time frame. With money being no object, cash readily available and you're patient, custom build has my vote. Custom builds tend to get very pricey and can take time, the best builders I know are months behind. There is another option for custom builds. Some rifle smiths will build rifles to place for sale online. It is a custom built rifle, but its not to your exact specs and it will still be pricey. Me personally, if I'm paying the same price I want it to my exact specs, but thats just me. Buying and modifying would be the way to go if money is a issue or if you had rather be getting some trigger time while you save up money. You can buy a ok rifle, scope it out with good glass, base and rings. Now you can be getting trigger time and practicing the basics plus learning other important things like ballistics of your caliber and reloading while you save money for better barrel, trigger, stock, and etc. So it mostly will depend on your budget and time frame of needing the rifle. 5R vs other rifling is like stated above can be debated for days. Having both, it has been my experience 5R was easier to break in (which breaking in is another debatable issue) and cleans up faster. Stock wise I can't not comment on the Magpul. I've handled rifles with the Magpul stock but not fired one. It does seem to fit me ok but that doesn't mean that it preforms. I have McMillan, H S Precision and Bell & Carlson. All three have met my needs and preform well. All three are also bedded and hand fitted by great rifle smiths. I have not researched the Magpul stock to see if it can be bedded and fitted. I recommend whatever stock you go with that you have it bedded and fitted by a rifle smith. Just my 2 cents, opinions and your milage may vary.
mckenziedrums had some very good points above about barrels. Its more important to get the correct twist rate for the caliber and bullet weight than rifling in my opinion.
