I had a .38 MKIV, I've also shot a total of seven MKIIIs, MKIVs & Enfields.
Things to look for include:
Is the locator pin for the extractor star snapped off? It will still shoot fine if it is.
Does the cylinder stirrup lock up tightly.
Rust.
Things to be be aware of:
The triggers are very heavy in double action and fairly heavy in single action.
The British service round was a 200 grain lead projectile, modern S&W rounds are 145 grain and will probably not shoot to point of aim.
The sights are rather crude, probably ideal for trench warfare but not for target shooting.
Ammunition is expensive and can be hard to come by. I sold mine after a year during which I couldn't find any ammo (I'm in Australia, so the situation will be different in the USA).
Webley made two MKIVs, the .455 MKIV of 1899 and the .380 (.38 S&W) of 1932. In between was the .455 MKV & MKVI. After WWII surplus .455 models were sold to the USA, often with the cylinder face cut to allow chambering of moonclipped .45 ACP rounds. This is a risky to unsafe activity due to the difference in pressure between the .455 & .45 ACP.