One of things that might be worse than "playing around" with loads???
Listening to about half the responses you get on this forum! Especially from those that seem to chime in like a flock of magpies on every subject offered up!!!

Sounds to me like you have found a gun that suits your needs/wants and you feel like you are getting it for a fair price? The RUM's are good shooters and now you just need to find a good load for it. Never experienced the 7mm RUM, but the 300 and 338 RUMS are nice and I expect you will find the 7mm to be likewise.
Remembering my comment earlier and that free advice is worth what you paid for it.....I will offer up this tidbit....if it comes with a muzzle brake as a lot of the RUM's do, take it off and put on a thread cap and never look back. The recoil of any of them is manageable, the noise level is not. Shoot it once without hearing protection and you have damaged your hearing. But it's your hearing.

Good luck on the hunt and report back on your experiences with the gun and the hunt!