The Ruger Old Army was Americas attempt to provide a rugged muzzle loading revolver utilising modern materials and engineering innovation. Apart from the normally fitted fully adjustable sights, it uses coil rather than flat springs which are the part most likely to fail on the Italian reproductions. The Old Army is based on a fine range of powerful breech loading revolvers and should provide a lifetime of trouble free shooting. The downside is that it is a heavy revolver that doesn't easily lend itself to single handed, precision shooting and is ineligible for some formal competition. However, if you just want to shoot it informally or in club competitions it would be a great choice. Unfortunately, the Ruger is no longer in production and consequently spares are not easy to source especially here in the UK. I've owned mine for over twenty five years and have never experienced a single breakage or reliability issue.
Finally I've seen many on offer at around the £500 mark, so in the condition you describe, that would seem to be a realistic price to become the proud owner.
Brian