Here's how I evolved from extracts, to a super-ghetto 3 vessel, to eBIAB.
Extract: Typical extract kit from Midwest. I had two 4gal pots used on a gas stove for heat. I did full volume boils.
BIAB: I used the above, with paint strainer bags. I started with 2.5 gallon batches. It was easy to use
this site to calculate how much water and grain would fit. I'd dunk sparge in the second pot, mix the runnings a little, then bring both pots to a boil. I started upping my batches to 3 gallons.
BIABucket: I used an old 5 gallon bucket instead of my four gallon pots because of larger grain bills and/or batches. I still used my bag to lauter.
Bucket-tun: I put a bottling spigot on the bucket. I made a CPVC manifold that attached to the bottling spigot instead of using my bag to lauter. Then I got a 6 gallon bucket. I ended up doing some batches up to 5 gallons on this set-up. I still boiled on the gas stove in two 4gal pots. I once got 92% efficiency on a session beer with this setup doing a fly sparge. I used a second bucket with a spigot as HLT and drained it into the MT over a plastic lid with lots of holes drilled in it.
eBIAB: I decided that fly sparging took too long and all the necessary buckets and pots strewn about the kitchen for extended periods of time were not good for my marriage. I got a 15gal Bayou Classic, a 5.5kW element, built a 240v panel with PID, a pump, sewed up a larger bag, and have been doing this for 3 years now. I have been doing 5 gallon batches since I got kegs, but previously did 5-8 gallons batches split into multiple beers (see diversity link in sig) on this setup.
Until the last step, it was pretty incremental. I made an immersion chiller near the beginning, and didn't change until the electric build. I added something like a SOF chiller for fermentation somewhere along the way. I made a stir-plate. I got an oxygen regulator and stone. Something for a birthday here, a Christmas present there. I still like smallish batches, and may go back to some more of them (getting too fat on 5 gallon batches and like to brew a good variety of beers).