Hello and welcome to the board!
You've done some excellent research and have some very good questions. Have you spent any time on our Slow Travel website? We have a page on
What is Slow Travel? which includes great articles on how to search for a rental, questions to ask, etc.
I took a look at the Friendly Rentals site and the apartments do look lovely. I took a look at the rental agencies and reviews on Slow Travel but I didn't find any reference to this company. But don't worry. It just means that none of our members have used them and submitted a review.
It think it is great that they are list their business registration. I think they want to reassure potential customers that they are a legitimate business. I'm assuming this is a real business registration in Spain.
E160 per day is a good rate especially for a large city. I don't think it is especially low. Definitely look at what neighborhood it is located and the square footage of the apartment. These can both factor into the prices.
Yes, it is very normal to pay a deposit by credit card and the remaining balance in cash. In fact, you are fortunate that they will accept the deposit with a credit card. Some rentals will request a transfer which will increase the price due to bank fees. Credit cards are expensive for businesses and owners will want the balance paid in cash to save 3-5% in fees from the credit card bank. It can be a challenge coming up with the large amount of cash needed but there are different strategies such as taking multiple ATM cards for withdrawls, travelling with the cash, etc.
My opinion is these apartments look great and it looks like a good rental organization. You can also ask the company for some people you can contact for reference as a final assurance.