I've been running variations on this build for some time (some builds posted previously in this thread), but I've found the following to be the absolute best
for me.
DRG-
FLAME
Endo Steel
Double Heat Sinks (14 total, inc. engine)
336 armour
RA - PPC, Medium Laser
LT - Gauss Rifle
LA - 2x Medium Laser
I used to run ERPPC's, but the heat generated was too difficult to manage effectively. Once fights got close, I was finding I had to simply stop firing them entirely. However, I really wanted the PPC's in there because particularly at long range I can never hold my lasers (I tried Large Lasers too!) on a single hit location for the entire burst. PPC's run nicely alongside the Gauss rifle to hammer single target locations at long range for substantial damage. The PPC is on the opposite side of the Gauss Rifle, so losing one from heavy damage to a side doesn't cost me both, and to allow me to fire either if I'm poking partially out of cover to either side.
In this configuration, heat is entirely a non-issue. It's possible to overheat, but takes sustained firing of all the energy weapons for quite some time, and the reality is that once you're very close the PPC is less effective, so you can lay off it a bit in those circumstances.
I used a variety of engines in my builds, and while I really appreciated the extra firepower I could get mounting a smaller engine (a lot of my builds used a XL255 I had sitting in my garage) I have to say I get vastly better results out of more speed, so the XL300 is IMHO the smallest I'll use from now on.
The Gauss rifle is critical to the build's playstyle. The idea is to hammer hard at long range with single shots "from nowhere" with the Gauss and PPC: Move to a firing position, strike hard with pinpoint accuracy, then
move. Do NOT fire and wait for a response, or try a second shot, just ****. Get a new position, fire, ****. Do this until the two sides meet in combat, then stop being a punkass sniper and switch to a quasi-jenner style skirmisher. Run in, hit fast and hard, and keep moving at top speed. Ideally, you peel off slower heavies disrupting their focus fire, but even without the speed allows good rear shot options, and a 40pt alpha can really put the hurt on rear plating.
I'd love to try an XL350, but I'm too poor at the moment, and have no idea how I'd manage it. As I said above, the Gauss is pretty critical to the build - heavy damage, low heat to supplement the energy weapons for best DPS:heat ratios. Could swap it for an (U)AC/5.. but it's just not the same. The *AC/5's require more time-on-target, making them poor for hit-and-run attacks. Probably go to Gauss+4MLas (ironically, where I started) ... But I wouldn't do it happily. Going to 4xMLas would gut your hammer damage; and require more dancing rather than in and gone.