You don't want an XL engine, because it makes the mech much more fragile.
Jump Jets makes your mech a lot more mobile. Problem is: There are only two IS assault mechs with jump jets:
- Highlander
- Victor
The Victor has useable jump jets and is light enought (80t) to get some mobility without an XL engine. But the problem is: The chassis is not very good.
Here is the best version I found, which fitted your description:
- Mobile
- Jump Jets
- Brawler
That is as good as it gets. If that is fine for you, good! Otherwise you have to give up mobility or robustness.
For a brawler robustness is very important. So you would have to give up mobility. Then there are a lot of mechs to choose from. As the people before me already told:
- Atlas - There is a ECM version around!
- Banshee - Good mech with good quirks
- Battlemaster - Good quirks too.
- Mauler - Exceptional good for AC
- King Crab - Even more AC, when desired.
- Stalker - With the Misery you get a ballistic hardpoint. It's considered as one of the best hero mechs.