Linux shared hosting is fine. If you have the affiliate website made in dotnet then go for windows shared hosting.
Don't really understand your question. There are tons of hosting companies out there that guarantee up-time and have quality support. The content is the important element for an affiliate site to be successful. Good luck
In hostgator I see they suggest me to add Addons "SiteLock" with web hosting plan. Is it essential for affiliate site?
Take a look at site5.com , they have great hosting solutions and great support. Also if you are looking for a good ecommerce platform I would recommend Shopify, it has a great UX and has apps you can install for affiliate marketing. Cheers!