Regardless of whether it's an MTB, a tourer, a Spa tourer, or 26" or 700C wheels, for a Shimano 9 speed triple set up the lowest gear possible will typically use:
- a 12-36 cassette
- a 'Shadow' version SGS/long cage rear derailleur (the Shadow versions are designed to cope with the 36t sprocket, the non-Shadow are designed for up to 34t) - use with either road or MTB shifters
- a road triple with 24t inner ring (smallest possible with 74 BCD) - needs a road front derailleur and STIs or road flat bar shifters
- an MTB triple with 22t inner ring (potential downside is a wider Q factor, which some dislike) - needs an MTB front derailleur and MTB shifters.
So in the case of probably any of the 9 speed Spa tourer options, Spa have the Alivio and Altus Shadow SGS/long cage rear 9 speed derailleurs in stock. At the moment they are showing out of stock for the 12-36 cassette, but they hold back some stocks for complete bike builds, so might actually have them. Even if Spa could not supply one right now, you can buy them elsewhere, e.g. Bikester (
https://www.bikester.co.uk/shimano-aliv ... 7767.html?).
With a Spa TD2 triple chainset and a 24t inner ring, your choice would be a combination of 46 or 44 outer ring and 34 or 36 middle ring, e.g. 46/36/24.
Personally I would not have a bigger difference than 22t between inner and outer, so if a higher top gear was required I would use a 26t inner, 38t middle and 48t outer. That gives a 109 inch top instead of 104 inch with a 46t, as illustrated in the link below.
http://www.ritzelrechner.de/?GR=DERS&KB ... 6&UF2=2170