...I encourage you to read the following R2R articles that will help answer your questions about set up and cycling:
...40B are a great flexible tank so I think they will be a good platform for you. There are several DIY 40B threads here on R2R that will give you a general ideas and others will likely pop into this thread soon to add additional comments. Good luck with your soon-to-be new system!
...but you will get some shading and loose par further out you get. No exp on the Kessil, but I really want to try them too.
You may want to check into T5 hybrids as well.
On your tank you may be best hanging the fixture from above. We got some lighting gurus that may stop by.
...problems that may come with the mods (yes, I have drilled and modded many tanks). IMO, the second option would be to clean it up, get some good pictures, sell it at a profit, and then buy another tank with the layout that you would prefer. Good luck with whatever you decide! :)
For test kits, I have found the most cost effective are Salifert. They'll last you a while too and should cost you less than $50 for the four you need (Alk, Calc, Mg, and Phosphate).
Skimmer, you can certainly pick them up used and save a significant amount...
First off #WelcometoR2R !
Let me disclaimer my response with the fact that I do not use Triton, but another method very similar. However, I think I can answer two of your questions..
#1, you should not need to wait 30 minutes between dosing as long as you are dosing into a...
Assuming that nothing is tore up, the glass isn't all scratched up, etc. it sounds like a very good deal - like @crabcore mentioned a leak test is a very good idea.
...and Welcome to R2R!!!