Well, sort of. The W*hr rating depends on the load. The more current you draw, the higher the internal losses and the less total energy you get. Also a 12V nominal battery doesn't provide enough overhead to regulate the 12V rail, especially since the battery is considered "dead" at something like 10.2V.

Lead-acid is really not a good choice for a portable computer. There's a good reason portables started out with NiCd, migrated to NiMH, and finally to Lithium cells. That SLA battery you posted probably weighs 10 lbs and may not even fit inside the machine. All of the above pack a lot more energy into less size and weight than SLA. You really do have to be careful with Li though and use a protection board and intelligent charger. NiMH is readily available, inexpensive, and relatively tolerant of less than ideal conditions.

-5V is easy, you can get an inverting DC-DC converter for a few bucks that will turn +12V into -5V, it may in fact only be used for the serial ports, I don't know.