pond construction questions.

Hello all,

I have a few questions and hope someone might be able to help.

I have a Beckett pump, forgot the rating, but its outlet is 1.5 inch.
I've read that you need to push the water through 3 inch pipes. So if
I use adapters I can go from 1.5 to 3 inch. Would this be ok for the
pump, would I gain anything from doing this or should I just leave it
at 1.5 inch.

Home Depot sells 20 mil PVC liner. Is this any good. 17x18 for $169
Can I get something better and cheaper.

A pond store guy told me that instead of a bottom drain, I should just
run a pipe to the deepest part of the pond. The end of it would just
be a strainer/grill that prevents fish from getting sucked in. This
way I don't have to put the drain through a liner and worry if it
leaks. Any opinions/advice about this.

Thanks in advance.