Welcome to the forum Tom, my advice would be to have a read through some of the build logs as you will learn a lot just by doing that & probably answer some of your questions as well.
The nema 23 motors will be fine so will the power supply & drivers although you might want to think about upgrading the power supply & drivers at a latter date, better off running the motors at 70v rather than 36v.
Your budget should be fine as you already have the electronics sorted for now.
Can't answer your question on what you would pay for gantry side plates but there are guys on the forum that will machine parts for people so just ask for a price when you get to that stage.
Problem people have with steel is keeping it square & flat especially when welding which will distort it making mounting rails more difficult. Some people use an epoxy coat to achieve the flat surface they need. Some people will build the frame from steel but then use aluminium extrusion or square tube for the table & gantry.
Profile rails are much better than supported rails but it comes down to price, they are more expensive & supported rails will still do a good job. Some people will opt for supported rail for the x & y axis & profile rail on the z axis.
At the size your planning 1610's will be ok & will give you better speed & feed rates than 2005 ballscrews.

Once you have read a few build logs then you can start a new thread with a design for your machine, best to design it before you start buying parts, once you have posted a design or the start of a design then some of the more experienced members will help you make sure the design is suitable for your needs before you start manufacturing it.
Hope that helps a bit, good luck with it all.