start by removing the interior so you can see the condition of the floors and inner sills, remove the arch and sideskirts to show the condition of the sills and wheel archs, get underneath and check out chassis rails and under the archs, the lower a pillars like to rot so do the rear inner archs, check under the bonnet, battery tray and fuse box area also inner wings, its worth taking the bumpers off as well to check underneath eg front crossmember, front panel, and rear panel, check the boot floor as well,

you can then see what your facing and order panels, I used these guys shipped to me within 48 hours they keep a good range of panels in stock

http://www.car-body-panels.com