With your 20 gallon kettle you could easily do a 5 gallon BIAB batch with only buying a bag. There are a couple of sites that sell them to fit that size kettle. You would need a way to raise the bag (I use a ladder over the kettle with a pulley and move it when I start the boil) because it can be heavy/awkward. I went this route to avoid having more gear to store.
I agree, although I regularly brew 5-6 gallon batches with my 1 vessel BIAB system. You could probably do 10 gallon BIAB batches with your kettle if you'd like. I usually check my recipes with a calculator from the Green Bay Rackers to see how much volume a batch will take.
I agree with the previous posters though, a bit more info is needed. Anyway, there's tons of ways to make great beer. I've used a 3 vessel system before, and it worked out well. I switched to BIAB because I found it easier, although every has their own definition of easy.
To answer your question, if you want to have a few vessels, and want to fly sparge and brew 10 gallon, this looks like everything you'd want for the most part.
- If you don't have a way to crush your grain, you might want a grain mill. I bought a cheap corona mill for $20, but I had to modify to get good results. I followed http://www.instructables.com/id/Corona-Mill-Mods-for-All-Grain-Brewing/ I also made a mill cover, because I was too stupid to buy a mill that came with one:
https://www.homebrewersassociation.org/forum/index.php?topic=29472.0 - Some grain storage. Once again, not a big deal if you have a great local homebrew shop. I have a hydroponics place that has decided to carry some stuff to avoid going out of business (I desperately try and support them to support competition), and a liquor store that sells malt for $3/lb. I'd recommend some buckets and gamma lids, or some vittles vaults. Petsmart has a nice sale right now for 65 lb containers, you can order online or they'll pricematch in store. Other places may pricematch too.
- Bru'N Water Spreadsheet https://sites.google.com/site/brunwater/
This is a fantastic way to make sure that you're staying in the right water parameters. I don't do a lot of water adjustments, but I do try and manage my pH. Probably pointless, as I don't have a pH meter, but I feel better knowing I'm listening to a PHD about my water. Even if I can't trust my local water report. - If your kettle doesn't have volume markings, you can make your own. I did, and I love it.https://byo.com/stories/issue/item/3113-etch-your-kettle-projects
- A chiller. It's a great time saver, but you might already have one. I like my immersion chiller personally, for reasons Jamil Zainasheff outlines here:http://www.mrmalty.com/chiller.php
I can't really think of anything else right now without more info, but those are my initial recommendations.