What's a good, reliable mountain bike drivetrain (1x10?)
I won't suggest any brands but here are some ingredients that will allow you to go cheap and reliable:


go 1x9. 2x is more expensive, more noisy, heavier and generally problematic if you can get away with 1x. Just make sure you calculate some possible ring and cassete options so you know that you'll be able to tackle all ascents and descends you need to. A spreadsheet works great for that. Divide ring teeth to cassette teeth to find the ratios that you need. Ride friends' bikes on ascends and descends for find your numbers.
bashguard: Do you really need it? Please check on your existing one whether you see impacts. Have you used it so far? If not, you don't need one. Especially now that you'll go to a single and smaller ring, chances that you really need one are very slim.
chainring: Go with a "narrow-wide" one. There are at least 5 companies making such rings now. The narrow wide pattern of the teeth allows for maximum chain retention eliminating the need for a chain guide.
rear derraileur: Definitelly go with one which has a clutch mechanism. It does work wonders.
crank: You'll most probably be able to use your current SLX crankset, but I'm not 100% sure on that. There is a possibility that you'll need to do some research and play around with spacers in order to have the perfect chainline so all cogs of the cassette are working well.
chain: If you go 1x9 get a 9sp chain. If you go 1x10 get a 10sp chain.