Yes... there are lots of combinations... which is a good thing... Higher end tripods like M/F and Gitzos offer lots of options for legs and head combos. I like the choices...
Tripods are the steadiest when the center column is all the way down. If you shoot mostly at eye level make sure the legs can be extended to that level.
The screw hole on the bottom of most SLRs is standard. My advice is to get a head that accepts a quick release. With out a quick release, using a tripod can be fustrating. The screw sizes to attach the head to the tripod legs comes in two sizes. Many heads will fit one or both. I have bogen legs with a Giottos ball head screwed on for example...
Make sure whatever combination you decide on is rated for the amount of weight you will place on it. This is especially true for ball heads as they generally support less weight than a 3-way pan. I almost ended up with a pistol grip style bogen ball head until I found out it is only rated to 5.6 lbs. Don't trust the statements that state "35mm to medium format". Base it on the weight of your equipment to the rating of the tripod legs and head. My Elan IIe+50mm lens is going to be a whole lot lighter than my EOS 1v+70-200mm lens...
Is B/M worth more than Velbon?.... only you can decide. I used to have a Velbon... and I would say the B/M is worth it. The biggest difference I've seen is the tubes used to manufacture the legs and center column. Thicker tubing is going to be stronger and provide a steadier base.
Specs for B/M and Gitzo products is here..
www.bogenphoto.com
I like Gitzos too but I found them to be more expensive. Carbon fibre would be nice... but VERY expensive... never could convince myself
As I said, there is no reason to spend big bucks on a B/M tripod. If you look hard enough you should find a used one for a reasonable price. They are built well and very simple pieces of equipment... not much to break. One thing to note. Bogen/Manfrotto reuse model numbers year after year but make changes to the design. I have a full height old 3011 legs. The same specs as the current except I don't have quick leg locks.
3001 series are very popular and built very well.