It will help to know their regiments, ranks and service numbers but, unless they were officers, or served in the Army before the War, you may be disappointed in finding their records, as only about 40% survived the fire in 1940.
You can search Documents Online in the reading room and print off what you find from medal cards for just 20p a sheet, rather than the £3.50 it costs to download them.
You can also search the catalogue before you go to see if you can locate any service records, which will give you the piece numbers you need to request (these will probably be original documents).
Although you are unlikely to find "other ranks" mentioned by name, it is still worth looking at the war diary for a soldier's unit if no service records exist, as it will tell you what whas happening at the time he was killed. If you are VERY lucky (as I was, twice!) you may just find your man mentioned.
If you have a half-decent digital camera, capable of taking pictures in a well-lit room without flash, take it along with you - you will be able to photograph any documents and save the cost of photocopying. If you have a laptop or hand-held computer, that is OK as well, but you will have to take it out of its case before going in.
Don't forget, you are only allowed to use pencils in the archives (no pens), and no erasers. You can take a notebook, but not a loose-leaf folder, and any loose sheets must be stapled together, so don't take valuable items like original certificates, etc.! All bags have to be left in a locker and the items you take in put in a clear plastic carrier (provided).
If you don't already have a readers' ticket for the NA, you can register on line before you go; they will take your picture and produce your card for you on arrival.
There is a car park at the archives, but space is fairly limited - there is no charge for parking.
Good hunting
Bill