a three in drop on most any car is a bit absurd...
on the air...
you can get an air bag-over-strut design...
similar to a coilover in design, it would fit in a civic with no probs... pretty much straight bolt in. you can get six inches of lift out of it...
easy street (air lift) has a kit comin out for civics... they have one that tucks the rims in the front and gets roughly six inches of lift...
and no body mods, other than for wheel clearance...
the susp parts will fit...
on the bracing, if the wheelwell is rusting, it doesnt mean it will fail. it depends on the depth of the rust... i would have a pretty knowledgeable shop look at it (ie racing shop) if you are going to rebuild your mounting points, b/c there is a lot of geometry in doing that.... if you are 1/8" off, you could destroy your whole suspension, and make it more dangerous than it was before you rebuilt it...
the forms and shapes in the metal as well as the thickness also add rigidity that you cannot overlook...
hope that gets you an idea...
and on air in cold climates, the air will contract and expand, following the temp.
hydros are oil based, will not freeze, and the fluid does not expand and contract. it also gives you a stiff as hell ride. the air bags will handle pretty good, ride like a caddy, and be easier to clean if you spring a leak. no fluid dumps on your driveway...