Good cartridge the .300 Weatherby is. I've got a Sako 75 chambered for it and I've been reloading for it for about 4 years. Here are some random thoughts about it:
I neck size only using the Lee collet neck die. When brass gets too large to chamber (about 6 or 7 loadings) I throw it away and buy new Norma brass and start again. The Lee collet neck die is the greatest thing since sliced bread in my opinion. For seating I use the Forster Ultra seater. I measure for concentricity and these dies working together (along with the Forster Co-Ax press) produce very straight ammo which aids in the accuracy department.
I've had luck with RL22, H1000, H4831sc and IMR7828. Of these, IMR7828 has consistently been the best. It'll work with just about any 180 gr. bullet. It produces good velocity and good accuracy. If it's a reduced load you're considering, I've had good luck with H1000 and 165 gr. Hornady SST's. I use 87 grs, which starts these out at about 3050 fps and accuracy is outstanding.
My current favorite load utilizes the 168 gr. Barnes Triple Shock bullet and RL22 (IMR 7828 works well too). This has been an accurate combination and on game it is devastating.
I've tried the Swift Scirocco and while it's pretty accurate, I don't like it's performance on game at all. It expands way too much for my taste. I like the Barnes TSX mentioned above best followed by Northfork, Nosler Partitions and Speer Grand Slams.
The only primer I've ever used in the .300 Wby is the Federal 215 (and 215M), so I can't really comment on using different primers. I will only say that the Fed 215 seems to work really well.
Rifles chambered for Weatherby cartridges have quite a bit of freebore (unless it's a custom job), so there's virtually no chance you can seat a bullet out close to the lands. I've loaded rounds as long as the magazine will allow and I've loaded them to the length the reloading manuals suggest. I haven't noticed much of a difference in accuracy either way.
Well, that's all I've got time for. Hope this gives you some things to think about and good luck with the new rifle.