As I posted in the other thread:
All-grain or extract, shouldn't matter. The extract is exactly
the same thing you get out of a mash, and probably better
if you are a beginner at all grain mashing.
Body is most influenced by the yeast you use because there
is a great difference in the % attenuation. American ale
yeast gives you watery beers, British ale yeasts (and I mean
the liquid strains from Wyeast, not the generic stuff available
dry) don't attenuate as much. Look at the list at Wyeast and
pick one of the lower attenuators and see what happens.
The other thing that influences the perception of body
is sodium ion, you can add a small amount (like 1/8 teaspoon)
of NON-iodized NaCl to increase body.
Yes, higher mash temps give you more non-fermentables,
but the yeast attenuation is more important. See the second
question answered on this page for mash temp info: