Manifold and PEX to replace galvanized

I am trying to work on a "budget" for renovating my existing 1/2" galvanized plumbing with a manifold and PEX. I have read a good deal about manifolds and PEX, but have a couple questions.

My Master BA has a sink, toilet and bath (no shower, yet), from the manifold would I need to run 5 homeruns, or could I tee off of the cold for the sink for the toilet?

In the kitchen, I have a double basin sink, dishwasher and refrigerator. Do I need to homerun the fridge (for ice maker and water in door) and the dishwasher or could I tee them also?

Also, the house is supplied by 1/2" galvanized (I know, I know), I have been pondering on upgrading that to 3/4" poly, but I am worried that the increase in pressure will create leaks in the galvanized that is in place. Should I be worried about this?

Thanks in advance!
Drew

Larger pipe will not increase the pressure, only the amount of water (volume) the pipes can carry. I commend you for realizing you need to get rid of the galvanized pipes, but I suggest you contact a plumbing contractor to get an estimate on what it will cost to install PEX or copper. A complete refitting is really not a job for a novice.