Currently running fairly old 3×9 XT. I’m a run it into the ground guy so it is just about all due for replacement. If I decide to lose the front mech what do I need? New clutch mech, is deore OK? Will a 10 speed cassette need a specific free hub? Should I aim for 1×9 instead to save me buying as many new bits? I almost never use the big ring up front so thinking a fairly low gearing option.

absolute minimum
new chainring, 11-34 9spd cassette, cheap or bodged chainguide
you could look at 30T ring if you’re worried about climbing and don’t mind losing top end speed, depends how fit you are and how big/steep the hills you ride are.

Or 10spd clutch mech, shifter, 11-36 cassette, dinner plate sized add on sprocket if required, NW ring. (10spd cassette uses same freehub)

I’m going 1×10 with a 32t ring and and an 11-36 cassette when current 9spd drivechain dies or dry weather appears whichever comes first. Mate has just done the same, it’s taking a few rides to acclimatise to local stuff but he’ll be there soon, don’t think he’ll be taking that set up to the lakes tho (I’m not planning to either).

You can use the same mech and hubs for 10sp?

I built my Salsa with the same 8 year old 3×9 that was on the 5, which is terribly unfashionable. However, it needs to go up very steep things and do lots of road, so I can’t see a benefit from spending money on new bits.

It does need a bigger cassette though – top and bottom.

You can use the same mech and hubs for 10sp?

you need a new mech (and shifter) same freehub

I have 2 bikes running NW chainrings with standard rear mechs, slightly shorter chain than a 2x or 3x set up & yet to drop a chain. Terrain around here is rooty rather than Rocky though but I do ride steps on them without problem. Cheap way to try is just get a ring & take 2 links out of the chain

10sp cassette will fit on a 9sp freewheel it’s the distance between cogs that’s different not the overall size. But you could use your old 9sp stuff first to see if you can handle 1x

Oh and no chain guard at all

The Chain Reaction 1×10 bundles seem to have everything you would need for the conversion

You can use a 1×10 shimano mech on a 1×9 sram shifter with just a little spacer… Not a totally viable option for most people tbh but could soften the blow

I did it recently and am glad I did.

I got;

clutch mech (2nd hand XT)
cassette (2nd hand XT)
chain (new – cheap Shimano)
shifter (2nd hand XT)
32t N/W front ring (2nd hand Race Face)
single ring chain ring bolts

Doing it slowly – as in looking for bargains on STW – I got it for less than £100. I was in a similar position to you and would have spent too much money trying to keep 3×9.

32f 36r lets me climb all day long and I’ve very rarely wished I had a bigger ring at the front. This is on a 170mm fs though, coil forks, 2.4″ tires etc. On a lighter hard tail I think I’d be tempted by a 34t front ring and one of those huge rings (42t ?) at the raer.

I was really worried I was going to miss the extra gears. I’m yet to though. 1×10 looks better, there’s less to wear out / be replaced and the simplicity means you can just enjoy the ride. I haven’t dropped a chain or had anything other than great shifting.

your 9 speed mtb rear mech will work just fine with some 10 speed road bike flat bar shifter.

If it’s all due for replacement I would be taking the opportunity to go for this 1×10 business. Worth considering…

36-11t 10sp cassette

Doesn’t look like anyone’s pointed that out so far. Widest range available and best suited to running an extender ring if you so desire.

A deore (or other) medium cage clutch mech.

Since your buying a new one a deore one will be fine. No need for a long cage with 1x set up. Be cautious going down the short cage route with full suss bikes.

Extender ring and narrow wide aren’t essential but…. will prob make the difference between you accepting 1×10 or hating it…

I’m just about to fit 1×10 to a new (26″ wheel) build.
Best value looks to be:
SLX mech, XT Shifter and KMC X10 chain from CRC
I was going to go for an XT 11-36 Cassette from Merlin, but picked up a SLX off ebay for a lot less.
Narrow/Wide 32T chainring from Superstar.

And thinking of a 42T extender ring and Rad cage from Oneup, but might see how the setup goes as it is.