I purchased a 300wsm Tikka T3 lite stainless synthetic left hand last year and shot around 350 rds through it so far. I am getting good results with it - around .5 MOA out to 200 yards and ALL groups of my best load under .75 MOA. (probably about 2/3 of them at .5MOA and I'm definatly not ruling out operator error.) I love the rifle, smooth action, very good crisp trigger. plastic clip however :(
I cant comment on the ability to build it custom cuz I just don't know. For the price, you can't beat it with a stick!!! I bought a savage 22-250 and I am disapointed in it - wished I would have spent another 150 and got the Tikka. I have recomended the Tikka to many people.
I would however consider the 7mm WSM
for the simple fact of better bullet selection -- just a thought.