While aspirin can be given over a lot longer period than Bute, at least my vet does not recommend doing that without a periodic blood test and does not recommend it for long term such as you are considering.
Glucosamine is ok for on-going issues but if the horse is insulin resistant or cushings, it cannot have glucosamine. It can have Chondroitin/MSM and HA (hyalouronic acid).
My 23 yr old IR horse has hock and ankle arthritis. He's been on chondroitin/msm for three years without issues. The vet said it would be ok to add HA to the mix so I that recently. It has made an even bigger difference in how he comes out of his stall in the morning.
Whereever you buy from just be sure the product is pharmaceutical grade. It will be pricey but you won't use as much.
Hope this helps some