The best places to find the Info you seek is:
http://m4carbine.net/ or
http://www.ar15.com/. I will offer you the simple advice that both sites aren't very friendly to newbies who come on there and post the same question as 72 newbies before them. Use the search feature and I am sure you will find someone will have already asked the question before.
That said, I would discourage replacing the front sight base with a gas block. most original manufacturers use tapered pins to secure the front sight base and you have to know which direction to drive them out. Most of the gas blocks I have seen use either set screws or clamping bolts to secure them, neither of which hold as well as the pin setup. If you are going to use a scope with more than 3X with it, you won't even see the front sight base. If you are running an Aimpoint or Eotech, they are in you field of view, but really not that distracting.
As for mounting your Magpul foregrip, I don't know what type you have, they usually attach to a piece if "Picatinny" rail or a Magpul MOE handguard. A lot of folks love hanging a lot of rails on their rifle, if you don't have a need for them I recommend against it. There are a lot of handguards out there that are modular now, the MOE being one of them. They allow adding a few pieces rail as needed, instead of 4 rails that cover top, bottom, leaf and right of the handguards that then get covers added.
You just have to ask yourself, is he telling you the truth based on knowledge and experience or spreading internet myths?
