Based on the question and on the advice given thus far, the following attempts to provide more information that you may or may not want
First:
Get a hold of Thomas Bont's software for the construction of GURPS TRAVELLER STARSHIPS. He produced it before GURPS VEHICLE DESIGNER came out. I don't know if he still offers it, but every single ship on my website was made using that software. I've also made a point of offering people the opportunity to request ships for which I would then include in my shipyards page. See
http://users.adelphia.net/~alaconius to see what I mean.
Second:
Star Mercs came out well before much of anything else did in the way of GURPS TRAVELLER products. In it, there is a well thought out "Mass combat" rules like section that is not unlike that which is found in the original MERCENARY book for CT. I find that I like it far more than I do the original GURPS MASS COMBAT rules for anything that is reasonably high tech. In fact, I would consider that STAR MERC mass combat rules are better suited for TL's 5 on up, and GURPS MASS COMBAT rules to be better suited for mass combat rules for TL's 4 and lower.
The one fly in the ointment as far as I am concerned is the battlesuit examples provided for in Star Mercs as opposed to later publications of battlesuits for use with GURPS TRAVELLER.
GURPS GROUND FORCES on the other hand, is very strong in details for military life and for unit organization - from platoon level on up to regimental and battallion level. Much of the information contained in Ground Forces is consistant with the CT material ever printed - including material in the JTAS supplements as well as from FIFTH FRONTIER WAR boardgame.
My suggestion for you is thus:
Get BOTH books if you can. Get Star Mercs first if you intend to play at a "local" level for your players. One nice thing about GROUND FORCES however is that there are templates if you will, for the creation of military vehicles for use with your campaign. It is a modular approach to building tanks and the like. In addition, there are some prebuilt ships such as troop transports and the like - not to mention battlesuits
(better than those in STAR MERCS I'm sorry to say).
One last comment:
Ground forces gives some fast and loose rules regarding logistics for units. This is information that should have been available for use with STAR MERCS. Knowing how quickly a unit goes through foodstuffs and ammunition is one thing that few "Military roleplaying games" ever bother to explore. This is why I strongly suggest you pick up BOTH books.