I've spent the last several hours reading dozens of threads on buying a house

and since I didn't see my situation, I'm starting a new thread. Sorry!
The house we visited is currently split into two flats. We'd have to spend money to make it back into one house because we need all the space. I'm not sure how the "currently 2 flats" would impact any Eigenmietwert. We don't have any idea how much the conversion would cost. There are other renovations needed, but we could move in and deal with them over time.
I've read on EF that it's a good idea to get a professional such as a "bauleiter" out to inspect the property and tell us what we're getting into before we make an offer. However this is apparently a one-shot bid. The agent said that they'll take bids until X date and after that the seller will make the choice. There won't be a second round of bids. The visit we already made is the only visit we'll get, as there are "many interested parties".
The agent also said that we have to first meet with the owner's bank to discuss mortgages. Only after that can we go to other banks. Threads on EF say normally we'd get a packet of info from the seller's agent that we could then use to shop around for mortgages. Of course that didn't happen. We apparently can't meet with the owner's bank until after an offer is accepted.
Given those factors, we have no idea where to come in with an offer. The house is priced fairly given the amount of work that needs to be done, so it's no shock there are many interested parties. But should we offer more, and if so how much more? She said we can send an email with the initial offer. Based on what I've read here, even that is non-binding as it's not signed in front of a notary.
I hesitate with the fact we can't shop around to see what our monthly costs would be. How can I offer X if I don't know what that means on a month-to-month basis?
Has anyone else encountered such a scenario?
- one bidding round only
- no info packet
- must speak to owner's bank first
Thanks for any insight and advice!