As always on abrasives, go for the best. The extra life far outweighs the cost.
I get mine (Norton cheapies) from Screwfix or the good ones in the Hermes blue zirconia from CSM Just Abrasives as they do 60 and 120 grit too. They also use a plastic backing rather than aluminium, which is less trouble for catching edges and is also lighter, which is nice on my 9" grinder-gyroscope.
(Get a paper catalogue, their web site is execrable)
Two standard sizes: some of the 4" use the little ones but everything else 4 1/2" and 5" is the M14 standard thread. There are also a couple of different side-handle threads, if you're using a stand or an Arbortech guard.
On the dust hazard thread, spalted timber is one of the serious few dust hazards you should worry about. European timber is generally quite safe (apart from the simple dust hazard) and it's the tropicals that have the irritation risk. This is serious, but usually highlighted by reputable sellers of exotic turning blanks. Some people do report problems with European species though, cedars are one of them. The biggest European risk is from embedded moulds rather than the timber itself.
Although the notion of actual "cancer" from these dusts is still controversial, the hazard isn't. If you're sanding, wear a mask.
Axminster had their yellow Perform on clearance recently. Street list is about 30 for that or the Nu-Tool. The Clarke machine you list is near enough the same. Sealey will do a very similar machine for 120!
If you're trying to keep the cost down, then use the angle grinder with a 120 grit flap and go easy on it. It's HSS, the risk of burning it is low.
Scary Sharp won't do HSS in finite time.