Let me take a quick stab at this for you...
Initially for my first 20 batches they were almost all around 5 gallons into packaging or a tad over, around 19 liters. Then I had several where I brewed extra because..it turns out, I drink too much beer.
Then I had some guys at the office, including my younger brother wanting me to brew beer for them too or let them by some of the excess if I maximize the batch size. I've agreed to a large extent. I'm not a huge IPA fan but do like them when well balanced so brewed 5 gallons for the brother and kept the remaining 2.2 gallons for myself. You might have no interest in agreeing to things like this, but it is easy to happen if you have the ability to do so... So for yourself or others, it is nice to be able to brew bigger than "normal 5 gallon / 19 liter batch sizes.
This is one where it's best in my opinion to get bigger than you think that need. I have a 16 gallon / 60 liter pot, and am glad it's no smaller than that. For beers with OG around 1.052 that finish around 5 to 5.5% ABV - I can often push it to around 8 gallons / 30 liters or so VIP (Volume Into Packaging), max. If I was brewing a larger OG IPA or Double IPA (DIPA), it gets more challenging to maximize batch size. As noted in above paragraph I made a DIPA and got just over 7 gallons but did some withholding water and extra sparging of grain left over, along with longer boil time... Kind of a mess. Extra work!
If you have a 50 liter / 13 gallon pot, you should be okay with DIPA batch sizes, VIP in the 19-20 liter range without extra work to go larger. So if that is acceptable to you, you are probably fine. Bigger pot gives you greater flexibility in larger batch sizes, or larger gravity beers.
Combo Kit w/ Burner and Bag
No idea what it all is in your kit. And sometimes products available in US and U.K. could be a little bit different. It's easy to get custom made bags of high quality custom tailored to your pot. So worst case if the bag wasn't good you could always get a better one later. Kits make purchases easier, so hopefully components are good.
No idea on your recipe... I use the BCS Book, Brewing Classic Styles, and we know how to convert this using our BIABacus file. If you had dimensions internal on the pot you could fill out a BIABacus file and post it in our Use this thread to convert recipes" thread, in the BIAB for new users section. Others can help us out there that have more experience and know Greg Hughes recipes. viewforum.php?f=5
Also on the above link - for sure download
BIABacus File: viewtopic.php?f=5&t=1869
Clear Brewing Technology (CBT): viewtopic.php?f=5&t=2685
CBT helps it all to make sense. And I hope the above is a help.