Couple of points. First, the Mesa M3 combo is definitely heavier than the Walkabout 12 combo, unless you lifted a unicorn (which is possible due to varying wood weights). Also, the Walkabout eq section is a unique thing of beauty and takes time to get to know. Having said that, the WA is more old school and the M3 more modern; maybe that's the rub. Lastly, I hear you regarding the bargain of buying a combo. However, the cost is flexibility down the road and IME that'll cost you more in the long run (ie-you want to change the cab, but have to dump the whole combo, or some similar situation).
With your budget, a used Markbass LM2 or Shuttle 6 head and a nice Epi or GB cab are within reach and IMHO will faaaaar exceed what you'll get in a new combo. Just my .02
With your budget, a used Markbass LM2 or Shuttle 6 head and a nice Epi or GB cab are within reach and IMHO will faaaaar exceed what you'll get in a new combo. Just my .02