Sounds like you'll be fine. Don't pay scads extra for the mum, nor the sling at this point. "Real" mum rifles (actual battlefield bringbacks) tend to be in worse condition for some odd reason, so that tends to balance out the lack of the mark on "pretty" rifles.

Slings? A whole collection can be made just about those! For you, esp. if you intend on some range time, it's not going to add a whole ton of value- its just a very fragile bit that probably needs more attention and ongoing maintenance than the rest of the rifle. (I've seen a bit of rope in a really late rifle add $300+ to its selling price - and it went before it's cheaper ropeless brother sitting on the shelf next to it! That's a little mad to me, even though the rope looked "correct".)

The bayonet, however, if its roughly the correct vintage as the rifle, can add $75-150 pretty easily. Do some study there before paying the premium, though!