For ziplocks, I've found that it's definitely worth going with Ziplock brand freezer bags (not the sandwich ones). They're much sturdier, and seal better. And be careful not to overfill. I usually leave a plate under a thawing one, but to collect the condensation as it thaws, not for leaks (I live in a very humid climate).
Re browning - browning things like meat and onions does make a difference in taste. If I were freezing stuff for later cooking, I'd probably go to the effort of browning the meat and onions, cooling it, and then freezing it in that state, particularly if I were doing a big batch at once.
Browned/carmelized onions actually freeze very well. You can slowly and gently brown a big batch of onions, divide it into portions, wrap and freeze.
One trick I use for freezing stuff for use in cooking is to take things like tomato paste, or pesto, or roasted red peppers and freeze them flat in freezer bags, fairly thinly filled (1-2 cm in thickness). Then, when I want a tablespoon of pesto for a soup, I just have to break off a bit from the slab.