How hard is this type of job?
It depends. Normally, it's pretty easy. But you can run into situations that make it much more difficult. For example, the opening may not be quite the right size. That's much harder than if it is the right size. Since you're removing an old one, it is unlikely that you will have that particular problem.
If the opening is too large, use Durock (aka concrete board) behind the surround to make the opening smaller. It's like drywall, but made from concrete.
If the opening is too small, you're going to have a much more difficult time. Basically, you'll have to figure out a way to cut the surround. This can be very difficult with the 3-piece fiberglass surrounds. 5-piece units are more adjustable.
Assuming you have a standard 5' tub, it is extremely unlikely that the opening is too small, especially if you're tearing down to the studs. It is much more likely to be too large.
Would the tile shaped make it harder to keep clean?
Very slightly. The tile pattern isn't nearly as deep as real tile, and the "grout lines" are much more rounded, causing the surround to drain more completely. That means far less mildew problems than real tile. But smooth will be just a hair easier to clean. Hard water will not have an effect.
Also some of the surrounds are 5 piece and others 3 piece, to my mind the three piece would be easier to seal, any opinion on which type would be better?
The main benefit of a 5-piece is it will fit in a wider variety of openings - you've got more places where you can tweak the size. Tubs have been standard for a very long time though, so it is likely the 3-piece will fit just fine, especially if you are taking a 3-piece out. The three piece indeed is less likely to leak, because there are 2 fewer joints.
One tip most people don't know: Do not put the surround directly on the tub deck (The deck is the horizontal surfaces at the top of the tub - where one often puts shampoo bottles and such). The tub will have a small flange that goes up the wall. You want to overlap that flange with the surround, but leave a 1/8"-1/4" gap between the surround and the tub deck. This is because tubs move up and down, depending on if they're full of water and people or not. That gap will give you room to put in some silicone caulking to seal the bottom of the surround to the tub. Too small a gap and you can't get enough caulking in, and the weight of the water+person will open a gap.
Edit to add 1 other really important tip:
Do not use drywall screws on the tub surround. Drywall screws have what's called a "bugle" head. They are designed to drive the head below the surface of the material you are screwing through. That will
easily split a fiberglass tub surround. You want to use what's called "pan head" screws. The back of the screw head must be flat.
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