I would bet they're going to go low-profile -- the Mike Vrabel strategy, i.e. see if someone else's castoff can turn it on here. A few who might fit that definition:
Jacob Ford and/or Dave Ball, DE, Tennessee: Both are physical fits for the OLB spot, both have had seven-sack seasons (Ball last year). Ford came from the same school as Markell Carter.
Stephen Bowen, DE/DT, Dallas: Bowen replaced Marcus Spears last year and played really well. He's sort of a bigger version of Jarvis Green, having a rep as a pass-rushing/disruptive 3-4 end who may not hold up as well as Spears against the run. If Dallas keeps Spears, Bowen is probably gone.
Antwan Barnes, LB, SD: He's undersized but has played the 3-4 OLB and can rush the passer a little -- he was really good against us.
Shaun Smith, DE, KC: He'd be even less exciting than Marcus Stroud, but he can play both end and NT in the 3-4. I'm not sure he's better than Brace, Wright, Deaderick or even Kyle Love, but he has been a starter on a 3-4 line in this league, for a playoff team.
David Baas, C, SF: The Niners have a lot of free agents and might opt to let Baas walk. He's a good center who could play guard for a year and take over for Koppen subsequently.
Personally, I'd rather they just spent it all on one Ray Edwards, or even Manny Lawson, but I don't see that happening. I imagine they'll make Mankins the priority and also try to tie up guys like Mayo and maybe Welker, then hope they get lucky with some bargain-basement signings. Another thing to think about: there are some good safeties on the market, including guys who have been good but are off down years (Dashon Goldson, Gibril Wilson), guys who have played on crappy teams (Donte Whitner, Michael Huff), and an older Pro Bowler (Quentin Mikell). We're pretty set at safety, but I remember thinking the same thing before they signed Rodney Harrison. Just a thought.