Why not just get stuff and have it fitted, both at your LBS?
Your old stuff broke probably due to age and perhaps mismatching of wear, who fitted it?
There really is no rocket science to this. Decent quality components (i.e. pretty much anything with a recognisable brand name) will not give you any problems if fitted correctly.
Its old parts fitted to a 2nd/3rd hand bike so probably was due to be changed anyway. This stuff is only easy if you know how or have money to risk mistakes, I don’t. I’ve looked online at different chains, cogs and chain rings and there is differences in BCD, Chain gauges etc Its the not knowing what chain will work with my XT cranks and the new ‘beefier’ chain ring/sprocket that confused me. I don’t mind buying my stuff from my local bike shop but they don’t stock ss stuff and would have to order it in especially and it would cost more than if i order it myself online. I want to learn to do upgrades myself in case of any emergency repairs etc I’ll get in touch with Charlie because he seems to come highly recommended for advice and sales.