if you get a twenty i would pay the extra $20 and get a twenty long over a standard twenty...i got a standard twenty simply because i could not find a 20 long locally and i would much prefer the 20L (which naturally showed up a month after I got started on the 20S)
i think your plan sounds pretty good. Those test kits will be critical, and as wayne mentioned, dont skimp on live rock. On a somewhat related note, to save a little cash, if you have an old HOB filter you can use it as a quasi-powerhead. Just remove the filter media and run it empty...does the same thing as a powerhead. Furthermore, you can add some LR rubble to the cartridge box and thus increase the amount in your display tank without cluttering the tank
as far as your hood, is this the standard black plastic one which comes with many tanks? if this is the case, depending on how you do your lighting, this may not work out for you. I know that on a 20 this is not a feasable option. You simply cannot get enough light through the one strip of glass on a 20. Maybe on a ten you could use a single 55w PC which should be sufficient for softies...all things to consider
What I would do is create an excell file (or something similar) with a shopping list of items you need, their prices, and the order you need to purchase them, and use that as reference. Will help you stay organized. Also, that way, if you have more than one option for how you think you will set the tank up you can compare them side by side