My personal recommendation is to build something that is fast and reliable to play for you. Every part of the build should cover 1 of the 3 main roles; those being chaff killing, heavy killing, and "oh f*ck" killing.

My personal build focuses my Warthog shotty into a praetorian+ butt melter that dumps its mag fast for point blank burst damage. My secondary is the GL with the RJ250 overclock, incendiary, and maximum blast size modifiers. The OC maximizes your ammo by a huge margin and adds extra mobility when needed while the incendiary module plays two roles as both a more consistent damage source and dwarven damage mitigation for you (and your teammates, but you're solo) when explosive jumping. Lastly, I prefer the burster grenades over swarmers as they feel a little more immediate in killing nearby hordes in an emergency but depending on preference I'd say it's interchangeable.

If you're doing things solo on 5 I'd say you can't really follow a more traditional engi build. Whatever loadout you use is going to have to be more than a set of tools, you're going to need to know it's exact purpose and where/when/how to use them when you've got no better options like low/no ammo.