IMO: This sorta thing is primarily appealing to people travelling from overseas who want to rent something they'll enjoy.
When I went across to San Diego during Christmas last year I paid $350US/day to rent out an E92 M3 which is roughly in the same sorta price range as the R35 (I actually did look for one but they didn't have em). This price included everything apart from fuel (only used about $20/day anyway) so insurance and 100miles/day (160km) were included in that price. Not only that - he didnt charge me any extra just because I was young (I'd just turned 20), he didn't give me any restrictions on it, he even told me that the US police couldn't give me any speeding fines so I could have my fun (and before anyone asks, this wasnt some family friend or weird benefit I was getting), he also told me that I didn't just have any specific time to return it on the return date - as long as it was within business hours. I did have to give him a $5000 bond tho (but i figured that was fair enough...)
To be honest this is the standard I would expect of a prestige/exotic car rental agency, having the M3 was one of the highlights of my USA trip. I'm aware that an M3/R35 costs almost double over here than it does in the states - but I think you should consider letting your customers enjoy the same benefits I did....