The third game by Activision under the MechWarrior heading was released in 1996; the first two were MechWarrior 2 and its expansion, Ghost Bear's Legacy. Mercs is not an expansion, but a stand-alone game using a vastly improved version of the original engine.
This game, in many ways, hearkens back to the very first MechWarrior game. Where the second game in the series did not limit the player in any real way (the only limit on what a player could take into battle was a mere weight limit), the first game was all about the money. Players had to pay for repairs, ammo, new weapons, 'mechs, and they could only get money through playing missions. Mercs brings back the challenge of playing in an economy, and presents an altogether more fulfilling experience than simply bashing your enemies with the best 'mech design you can cram into the weight limit.
Like MechWarrior 2, the plot of the game is fairly deep, but you must dig to find it. In this case, you are a mercenary whose previous boss got herself killed, and now you must fend for yourself while making your own name known. And so, you strike out, putting down a rebellion here, escorting VIPs there, assassinating a bank here, and so forth, until these unknown invaders from the edge of space show up and start kicking everyone's ass.
The rest is the finest part of the Battletech universe's history, and it makes for a wonderful backdrop against which the game is played. I dare not spoil it for those who don't already know it.
The gameplay enhacements from Mechwarrior 2 are quite numerous. For starters, 'mechs can now have their legs blown off and fall over. This does not instantly kill them, but it certainly quickens the process. Additionally, you can gain salvage after missions which will be added to your inventory; occasionally, you will salvage entire working 'mechs. There are also vast graphical improvements, most notably in the particle effects.
The original release worked in DOS and in Windows 95. The Windows version had a number of graphical issues and was generally less stable than the DOS version; the best way to play the game is on a PC contemporary with the game, running DOS. Although it will work on modern PCs, it will not be especially stable.
The music is important to note. It is similar to the music in MechWarrior 2, only where that music was done with violins and various other classical stringed instruments, Mercs' music is done with electric guitars, synthesizers, and other such things. And it's good; easily the equal of Mech 2's.
Activision later released a "Platinum" edition of the game, as well as a version 1.1 patch which turned previous versions into the Platinum edition. This version is only for Windows, and makes several important changes:
First and foremost, it adds support for 3D accelerators. On the plus side, the game looks pretty stunning; on the minus, it is unstable as hell.
Secondarily, salvage is dynamic. Previously, salvage was entirely scripted (beat mission 'x', get salvage 'y'); now, it depends on what 'mechs you actually leave bits of to salvage. Blast the 'mech to pieces, get nothing; carefully blow out the cockpit, you've won a free 'mech.
Perhaps most importantly (in some people's minds), the game no longer ends. Previously, when you beat the last mission, you "won" and the game ended. Now you can continue to play the random missions with your elite merc crew until you grow tired of the game.
When all is said and done, Mercs is far and away the best game of the franchise. It is well worth getting your hands on an old P75 for the purposes of this game. Copies of the game are slightly harder to find than P75s, on the other hand, so some searching may be required; it's well worth the search.