The good news is, we already have a number of letting agents using QuickFile, so I do believe this is suitable. We even have a guide to help you get started with the bookkeeping for this type of business (you can find it here: Bookkeeping for Letting Agents)
The way QuickFile billing works, is you pay by the number of nominal ledger entries in your account rather than picking and choosing which account size you want. So all accounts start as XS and gradually increase as more invoices, bank entries and journals are created.
This also means that if you wanted to sign up and give QuickFile ago, you can without any commitment. Regardless of account size, all new accounts have a 60 day free period from the date they were created.
If you specifically wanted the features available with the L and XL accounts, you can optionally purchase the Power User Subscription and add this to any account. The bank feeds are a separate annual subscription, so again, these can be added to any account.