offset is in the simplest terms, how the wheel sits basically. the smaller the number, the more it sticks out and gives you that wide stanced look.
4x100 is the bolt battern. all fits are 4x100 as are most older civics.
the mugen xj is a REALLY nice wheel, and like type said, its bascially been made for your car. the offset on it i believe is the same offset as your regular stock wheels...which means they wont stick out any further and def. wont rub.
mugen makes a VERY nice product, superior to O.E.M. stuff in most cases.
but, that quality and sought-after aura that surrounds mugen is also going to fetch you a very high price tag.
honestly, as far as wheels that I personally like go, i can think of any that exceed 1000$ PER WHEEL than maybe a few 18 or 19 inch bbs models.
the mugens are considered expensive at over 300 a wheel, but for mugen wheels thats not AWFUL especially considering is a brand new design.
anything by mugen, bbs,ssr, enkei,rays, volks, wedssport, work, yokohama advan or oem (original manufacturers equipment) will be very high quality, but it can be pricey sometimes.
some companies such as konig and rota make replicas of the more popular designs of these wheels, and they are rather cheap, but decent in quality.
these two companies have been around longer than some of the other knock off brands, so you could score a nice set of em for cheap.
Konig, Rota Dealer: Buy Low Priced Suspension, Interior, Exterior, Exhaust, Intake Auto Parts: JLB Motorsports, Corona
this is a good website to get them from, very cheap, brand new.
brands like ssr, 5zigen, rays and enkei have nicer models that are not super high end, but still are name brand quality products that you can get for cheap if getting a replica wheel is not your thing.
hope this helps!
(and dont go bigger than 17's.

)
OH!
to the O.P.: if you want lowering springs that increase the performance of your car, but can be more readily used with stock shocks, and dont lower to much...look into swift springs.
you can get them at
www.kingmotorsports.com