If your camera takes so long to process a long exposure then it will hold you back when trying to take lightning photos. The camera needs to have its shutter open for as long as possible so as not to miss any lightning strikes. Its mostly down to the processor in your DSLR and how fast it can preform. Also, in daylight, even if you stop the lens right down and use an ISO 100 or lower, you're still probbably looking at a shutter speed of less than 1 second. If you're serious in taking lightning photos, you'll be better off with an ND filter (you can buy them up to 24 stops!!!) to darken your shot and force the camera to keep the shutter open for longer.
As for exposure, if you insist on using manual then note the light meter in your viewfinder and make sure to shoot in RAW as the exposure can "pushed" and "pulled" later. OR, use aperature priority
For stormy skies, you may want to invest in some ND grad filters. These allow you to expose the sky to the same brightness as the land, giving an evenly exposed shot. I use Cokin filters, here's an example of a shot i took with 2 Cokin filters, a 121 and 121L
A tri-pod, low ISO, high f-stop and some patients (if thats how you spell it) is all you need to think about. KISS (Keep It Simple, Stupid)
Good luck
Jayson