[Closed] Fork & Chain Advice Please

Long Story. 3 Years ago converted my 2002 Rockhopper Comp to commuter by adding slick tyres and a rigid fork. Last year bought a Giant Rapid 1 on cycle to work so want to bring my Rockhopper back into the fold. Recommend me a new fork (not crazy expensive). Also want some advice on replacing the chain. How do I know what size chain to get? I realise this is probly a pretty basic question but I havent replaced it since I bought the bike. Should I change any other drive train components (derailleurs etc)