what are the dimensions of the air handler unit & coil?

that is where you start.

will return be located below unit?

I gave up a portion of my linen closet for my
ahu, supply plenum & return below.

fyi:
for my 2 ton vs ahu the closet is 2'x2' with little room
to spare, maybe an inch on each side & 3" at front of closet.

I call it a closet also.
be sure you make closet air tight, sheetrock inside,
seal all corners & tightly around supply plenum if
it penetrates into the attic space.
return air should also be air tight.

tell us more.

best of luck..

Check the specifications on the unit you will be installing for the minimum clearances on all sides.

And allow room for servicing if necessary - that will also be in the specs.