Could well be down to knackered cables, especially if the bike has sat around. You can fit new cables and have change from a fiver.
The shifter may well need a blast with WD-40 or similar, followed by a drop of lube in the cable port. As for the front mech, a clean and degrease followed with a dab of lube on the pivots will certainly help. Front mechs are much of a muchness. SLX will see you right, they come with the correct adapters to fit you frame.
My own P7 is 2×9, I run a 36/22 front ring setup with a Blackspire Stinger dual chainguide and a bashring. It’s very belt-and-braces, but the drivetrain is very quiet. The Stinger is dirt-cheap at under £25, so it’s worthwhile fit.