How to tell if an apartment listing is legit and booking is valid?

I am in the UK. I found an apartment in Germany listed on homeaway.co.uk and booked it. I had to create a homeaway.co.uk account but did not give any credit card or other payment details.

I then received two emails:

• one from homeaway.co.uk saying my booking is confirmed and payment is due on (date of arrival), without giving any information about how to pay. This says that the property is professionally managed by HRS Destination Solutions and has a website link and phone number for them.
• one from HRS which is mostly in German and includes a personal email address (looks like "full name"@web.de) and what looks like a UK mobile phone number. This one says I should ring the number 30 minutes before arrival to collect the key, and payment in cash on arrival.

Two main questions: 1. How can I tell that this listing / booking is legit? You hear a lot about scams in this arena and I was very surprised to be asked to pay cash rather than bank transfer or credit card. I would not hand over the cash until verifying that the key works in the apartment, but is there anything else I can do to protect myself? 2. Would I have any come back against HRS or homeaway.co.uk if it turns out to be a scam of some sort?

Although the listing looks legit (has a few reviews over a few months, etc.) something is raising my "scam hackles" here, probably mainly the request to pay in cash.