I recommend a 'MechGrinder scenario with units having Gunnery skills in the 2 to 3 range depending on BV2. It allows people to jump in and leave as they please, and if you get killed, you can jump right back in.
Rules: If your players are newbies, have never played BT before, or haven't played in years, then keep it simple. No crazy rules from TO/SO/IO, though introducing them to 'Mech Quirks or Special Pilot Skills can add a little variety, so long as you can keep it balanced.
As for 'Mechs, in these types of scenarios, I usually go for a mix of mediums and heavies -- about the 50 to 70 ton range. It offers a good mix of speed, armor, and weapons. Though we've run these scenarios with all assaults with great success. The point is to keep your forces as balanced as possible -- which is why I rarely mix Clan and I.S. forces in scenarios like this.
Maps: You can't go wrong with the tried-and-true Battletech maps from the older boxed sets. But if you have one of the new Battlemats, those are actually pretty nice to play on.
I also recommend using record sheets printed from SSW or some other record sheet generator that will print a smaller RS but will have all the tables/charts on the front of the sheet. Flipping pages all the time gets tedious.