With the PEX you will need to get the crimp ring tool made for your PEX tubing. I have never used Vanguard but you should do a search on it with goggle.
Are you planning on changing it all the way to the valves? Sometimes with old galv it can be a pain to get adapt to the old pipe. It would also be a place for leaks in the old parts of the pipes.
As far as a whole house manifold I hate them. I will put 4 port manifolds in a bathroom in the shower access door and use them for the bathroom but don’t like to use one manifold for the whole house. I like to just T off where needed.
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I am anti PEX because of the local codes where I’m at. But as a whole, PEX is a good product if it is done right. Where I’m at you have to support the PEX every 2 feet and it must run in straight lines. Most places it will pass as long as it looks nice.