Welcome to the board.
The way I would work out a pentagon tank is to divide it up like so...
You can then work out the amount of substrate for each area separately and then add the figures together for the total amount. (A+B+C=x)
A and B are simple rectangles to work out, then work out the rectangle shown by the dotted line and then divide by two to get the figure for C.
What is your kh value in your tank? If it is extremely low, then you will notice larger fluctuations in pH as you wont have enough carbonate buffers in solution for stability. If your kh is alright, then you wouldn't expect to see much fluctuation in ph IMO.
Many people have ripped out an undergravel filter for a new substrate but it's still not a nice, risk-free job to do.
The extent of disruption depends on how long the UGF has been in place, how often it was cleaned (underneath as well) and the amount of waste your system produces.
There could be a lot
of mess under the plates, or there could be relatively little.
How many gallons is that tank? That will give an idea of it's stocking levels at present and thus, whether the filter would be sufficiently matured in time.
If it were me, as you are laying a brand new substrate that will be lacking in rich detritus, I would put a thin layer of peat in the substrate to kick start it quickly.
Personally I have never tried any type of cycle aid product, these may be of help in getting the filter ready, or they may be useless... hopefully someone else will advise on this.
Make sure you have Ammonia, Nitrite and nitrate test kits when you change over to keep a close eye on happenings, just as you would when starting a normal new tank.