I personally think a ten gallon boil kettle is one of the worst size kettles (unless it's perfect for BIAB. I have no idea about BIAB). You need to size your kettle for whatever size batch you'll be doing. 5, 6, 10, or 12 gallons are the common sizes.
To calculate your bk size, first consider how much break and trub you leave in the kettle. If your batch size is 5gal and you leave a half gallon in the bk, you need 5.5gal post boil. You then need to consider your boil off rate. Wider pots typically have a higher boil off rate. Northern Brewer now carries kettles with an "optimized" height to width ratio. If you boil off a gallon per hour, your pre boil volume would then need to be 6.5gal for a 1 hour boil, 7gal for a 90 minute boil. You probably want some head space to allow for some foaming.
If you're only ever going to brew 5 gallon batches, I'd recommend an 8gal pot. If you think you'll ever be doing 10 gallon batches, go for a 15 gal pot or a decommissioned keg (15.5gal). I had a 10gal bk and had to upgrade when I went to 10gal batches.
As for mash tuns, I have a 48qt cooler can do a 6gal batch of really any gravity I want. I can easily do 10gal batches up to about 1.065 and have done an 11 gal batch of about 1.075. If you want to do large batches of high OG beers and don't want to supplement with extract (no problem doing that), then you'll want at least a 70qt cooler or kettle.