For a budget team, don't do a fuel cell. A fuel cell is going to be closer to $2k once it's installed. It's very easy to get shipwright's disease and want to replace everything but you might want to resist that urge for now. If it has been sitting a long time, you'll want to renew the soft parts (fuel lines, brake lines, brake hydraulics, etc) and flush the radiator Otherwise make sure everything is in decent working order.
Get the thing up, running and driving before you do anything.
When you strip it, leave all the stock wiring, don't pull it out and spend hours later trying to fix that one thing it needed to run. Otherwise, if it can catch on fire, remove it.
Have someone who knows what they are doing build the cage. Spend your money there.
Seek help on how to properly wire the kill switch.
You can often save on the seat by looking for a quality used one on Craigslist vs saving money by buying a new cheap one. Search for Corbeau, Sparco, Momo, OMP seat and make sure it's not a knockoff/no-name. You will likely come across aluminum seats from Kirkey, Ultrasheild and Butlerbilt for cheap money. If it has a single hole for the shoulder belts to pass through, it will need modifications to pass tech. The ones with separate shoulder belts holes are ok. Find the widest ass on your team and measure it. Small guys might need a 14" seat and the larger dude's 17" but you'll want to know that before you shop. You can pad out a big seat some but some of your team may not fit in a small seat. You'll want to have the seat before the cage goes in so it can be properly fitted.
Change all the fluid/filters and use quality brake fluid. If performance brake pads are available, buy them and keep the originals as spares.
Spares are nice to have but cost money. Cars generally fail in the same way with the same problems so do some research and find out what fails on your pile and be prepared for that failure to happen to you.
1990 RX7 "Mazdarita" 1964 Sunbeam Imp (IOE 2013 Sears Pointless) 2002 Jaguar x-type (Winner C-Class 2021 Sears Pointless)
Gone bye-bye
1994 Jaguar XJ12 (Winner C-Class 2013 Sears Pointless) 1980 Rover SD1 (I Got Screwed 2014 Return of Lemonites)