I am using my own rules that are an expansion of the MW1e system. With them, or with minor tweaks, I can make any character anyone wants to play. I have attached them for reference.
One tweak that would be nice to make is on the "Equipment" tab, cells A27 through G34. The power points per gram (kilo or otherwise) needs to be a little more consistent. For example, the Military power pack needs 20 grams to store a point of power, while the Micro only needs 1 gram to store 1 point of power. I'd recommend dropping the Micro Power Cell to only store 1 point of power, and increase the battery life of equipment that uses Micro Power Cells (i.e. radios).
On Weapons tab:
Line 51, the Pulse Laser Pistol says that it uses 2 pts of power per shot, yet a Burst that fires 5 shots uses 15 pts of power.
Lines 65-67: the 200g grenade and the 600g grenade both have the same range when thrown. Should the lighter grenade get more range, or does its small size actually cause greater wind resistance for its mass?
Line 83: the reload cost for the Night-Vision scope is 450g? Should that be its Weight instead?
Lines 95-100: does this list mean the gun can carry up to 6 of each grenade? Or should the number of shots be on line 95 to reflect that the grenade launcher has a single clip with a capacity of 6 grenades?
Armor & Combat Attire:
"Suite" should be "Suit" - lines 9, 10, 11, 14, 18
MW Combat:
A8 & G11: "Dodginig" -> "Dodging"
A10: "concelament" -> "concealment"
They want to have fun. My brother has played before, but the others are my nephews and grandchildren who are blank slates so they will simply see it as a Space Opera RPG, so anything goes as far as adventure. I plant to let them choose a character type to play and build the game around them.
Space Opera? Sounds like you should have them do special agent type missions, where they prep a location for the follow-up Mech force, rather than getting in their own Mechs. So they may have to sneak into the Mech Hangar and sabotage the Mechs, or put knock-out drugs into the Mechwarriors' food. They may have to shmooze at a party to get to know the people involved, and decide if the person who sent them on the mission was accurate, misinformed, or lying to them. They may have to sneak into the enemy base during the fight to shut down comms and capture the enemy leader, but they have to do it before their side's Mechs get destroyed or are forced to retreat.
I'd be tempted to pull ideas from the A-Team, and add a bit of merchant prince who makes sure the group can pay its bills at the end of each mission. So they may have to lie, cheat, and steal, but at least they got rid of the bad guy who was oppressing people