The level cap of 20 is an old AD&D tradition, which they raised to 40 in later iterations of the rulebooks. Considering how influential AD&D is on crpgs, it isn't surprising to witness a cap of 20 for Fallout 3. And frankly, by lv 15 I was destroying everything with Fawkes, had 1300 5.56 ammo, 800+ Laser Cells, Micro Fusions, 5mm and 300~400+ ammo of other types. The only ammo that's rare for me are explosives and .44. With Mysterious Stranger and a decent Luck, I felt way overpowered. This is on Hard. I can't imagine what's 20 going to be like with Action Girl and Grim Reaper's Sprint.
As for uniques, most (all?) of them can be repaired using standard gear. I'll list some -
Terrible Shotgun - Combat Shotgun
Lincoln's Repeater - Hunting Rifles (yes, it uses .44 ammo, but according to fallout.wikia it also accepts parts from Hunting Rifles)
Fisto! - Power Fist
Ranger Battle Armor - Talon Merc Armor or Combat Armor
XuanLong Assault Rifle - Chinese AR
A3-21- Plasma Rifle
I've even read that T-51b Power Armor can be repaired, but not with Enclave's Power Armors. You need to get hold of Brotherhood of Steel Power Armors, although I'm not sure if Outcast's armors will work as well. I would also expect Eugene and Vengeance to be repairable with mini-guns.