Start with the fork travel. The bike is set up for use with a 120mm fork, and so the start point should be to keep to that travel. The bike was designed to run with 120s and the geometry will be optimal. Add too much fork travel and you raise the front of the bike which will have the effect of making it vaguer, and possibly raise the BB and to a,point where the bike simply doesn't handle as well.
You've said you want plush, and would prefer an air fork. The choices at 120 are great, and so a lot depends on your budget. If you go second hand you'll find a good fork about £150, more if you want recent models.
Mainstream choices would be:
Fox Float 32. The 32 refers to the diameter of the stanchions. The thicker the stanchions, the stiffer the fork. Again, for glossary purposes the stiffness relates to the amount/ lack of flex fore and aft and torsional flex. Push a flexy fork hard into a corner and the stanchions twist and flex both fore and aft and around the wheel hub, affecting the stability of the wheel and causing steering/tracking issues. Fox forks are annecdotally flexible especially in 32 flavour. They are light, and have an adjustable air spring with reasonably adjustable compression and rebound damping. Compression damping can be adjusted to suit the terrain you are riding, the choice being a firm starting stroke or a plusher feel. Rebound damping is the speed at which the compressed fork recovers after a hit, and is adjusted to suit your weight and riding style. Fox forks tend to be dearer than many others, and have a short service interval of 50 or so hours of riding. You should be able to pick up a good used fork for about £200. There are variants of the Float with different compression damping systems, Open Bath, FIT, CTD etc. I had an open bath Float 32 in 140 flavour on one bike, and was seriously unimpressed with its performance.
Next are Rockshox. There are many different models, all 32 mm and available or adjustable to 120mm travel. Reba are my favourite 120 fork to be honest, but my experience is limited to pre 2013 versions. They have a dual air spring which means it has a positive and negative air chamber. You pump the positive chamber to a given pressure for your weight or chosen sag, then the same pressure in the negative. You can then adjust the spring progression rate between very plush with a linear stroke, to a firm starting stroke with a far more progressive stroke at the end of the travel by adding or reducing negative spring pressure. Rebound adjustment and compression damping can also be adjusted depending on the motion control damper or MoCo. The fork can be set to virtually locked out, but where it will react to big hits but stay firm for fast smooth trails, or opened so that it will be active all the time. They are lightweight, stiffer than Fox forks, and have a longer service interval. They are also easy to service if you are any good with the spanners. You should be ale to pick up a good dual air Reba for about £130-£150. Variant are SL, Team, and one or two others, all depending on the type of motion control damper fitted.
X fusion Velvet is a pretty simple but very well reviewed fork. I have it's big brother, the 35 mm stanchioned 160 Slant, and love it, but I can't help with a review ofthe Velvet. It could be worth googling.
I think Rockshox do a 120 recon gold air, but again, I have no experience of them.
As for compatibility, you have a tapered head tube, and so could fit a fork with a tapered steerer with your current headset. The steerer measures 1 1/8 inches at the top, and 1.5 inches at the bottom where it fits into the crown. This tapered effect supposedly reduces flex in the steerer. You could also fit a straight 1 1/8" steerer, but would need a reducer headset which either has a different size bottom head or crown bearing, or a system like Hope's which has an adapter ring to fit a 1.5 crown race to a 1 1/8" fork crown.
When buying a fork, ensure it isthe same length of steerer as your current one, or longer. On some bikes the steerer will have been cut too short to fit your head tube.
Ensure the axle dropouts are the correct standard for your wheels, unless your hubs can be adapted to fit different axles. The three standards are 9mm axles with quick release skewers (QR), 15mm Maxle, and 20mm Maxle. The two Maxle standards make for a stiffer fork. My Reba is tapered steerer and 20mm Maxle. It doesn't flex much!
Make sure the fork you buy has no damage to the coating on the stanchions. Pitting or scratches will ruin the seals and allow water into the lowers. Any scratch that you can feel with a fingernail means the fork is more or less scrap. The fork should have been serviced regularly, with lower splash back oil replaced according to manuf recommendations. Full service including damping oil and air spring seals will be needed if the fork wheezes or clunks when compressed. Any play in the lowers means the bushings need to be replaced.
