Normally something like a Beattie Intenscreen or Maxwell focusing screen has the reputation of giving you a brighter finder, but most photographers report that on their 67, it's creates even more vignetting and is minimally brighter in the center, therefore not a solution for you. How's your eyesight? Do you need a diopter for the viewfinder? If not, I honestly can't think of a solution with your current 67 other than bracketing focus.
67II? Pro: Solve your problem, can use your present lenses, maintain large 6x7 MF negs. Con: Expensive, the hassle of selling of your 67.
645N? Pro: Solves your problem, cheaper than 67II, respectable MF, 15-16 shots per 120 roll. Con: Hassle of selling your 67, need to replace lenses in competitive markets with 645D and 645Z owners, neg is almost 40% smaller than 67, the 75mm f/2.8 (equivalent to your 105mm f/2.4) won't have the same amount of shallow depth of field due to focal length, aperture, smaller format.
So in terms of cameras, you need to ask yourself to what degree can you afford not to upgrade? How critical is it to have the 67 size vs. a 645 size? Personally I love the 645 because I can shoot with it all day in the field without a tripod and they are much cheaper than the AF 645N....if you can live without AF. With my 75mm f/2.8 or 35mm f/3.5, I find the finder bright enough, although others with slower lenses do complain about the dark finder.
For developing B&W, I find that I can do it better myself, and cheaper, than any lab. Scanning, however, is more problematic. I am lucky because I have access to a MF film scanner (Minolta Dimage Scan Multi Pro), but for much less with only a little loss in quality, I've seen others use an Epson flatbed scanner (1/4-1/5 the price) with a MF film holder. For color, there is no magic bullet other than a great lab that prides itself in quality and treats your film like a national treasure.
My two favorite labs are
http://aandi.com/film and
Homepage ยป thefindlab.com
For anyone on a budget, neither are cheap, but I have learned the hard way that there is no point in sending your precious film to a lab that will disappoint you. It's the price of shooting MF and considering what it would cost for you to upgrade to a 645D, MF film is still a great value unless you're a working pro and speed is essential in your workflow.
If you've got more time than money, explore if there are any community colleges or universities with 'outreach' night classes with access to a lab and/or exceptional hardware.
One last perspective: There are limits to how good a scan anyone can get from a $600 Epson flatbed scanner vs. a $2500 MF Minolta/Nikon/Braun/Plustek/Pacific Image film scanner vs. a $10K Noritsu film scanner vs. a $20K Hasselblad drum scanner. Sending your film to a good lab with high quality control management will exceed the quality you can get with all the personal effort and time you invest with a prosumer level scanner and software.