Marauders shoot well out of the box. Mine has no modifications, only some tweaking to hit 830 fps (give or take, not regulated) with the JSB 18 grain pellets in .22. Even thought it does not have a regulator, it shoots extremely well. I've had the gun for a bit over 5 years now, and shot several thousand rounds through it. It's a fantastic starter platform, especially if you like to tinker. You can adjust the Marauder just like an FX Impact or Crown (except for the regulator). Very similar principles in adjustments. Great learning and shooting platform.
One of the strong points for the Marauder is the trigger. It's a solid, adjustable, two stage trigger so you can set to meet your needs. The Impact is better, but not that much.
As for modifications, I've seen people just about gut the gun, put in all after market parts, and really spruce it up. Or, you can shoot straight out of the box and have a good experience. Choice is yours. For me, the only modification I'm thinking of is possibly a Huma Regulator, other than that, I'm sticking with the gun as is.
Do make sure you have decent glass. My Marauder has a 4-14x44 on board, and it is quite adequate. The Impact has a 6-24, and to be honest anything past 18 is worthless. Just can't get lined up properly to use it. Lower magnifications allow you to scan for critters as well, so get good glass at a lower magnification would by my advice.
good luck, with so many solid choices out there, you really can't go wrong.