Hi Jo,
Marbella is still a sought after area and it is only in the last 18 months that the price drop has started to move to the area. Properties that dont sell within 6 months are normally over priced in the first place especially in todays market. If you like the place, make an offer and see what happens, if they dont accept you can move on to another, it is a buyers market.
Short term rentals - you will never be rich and dont expect to much, you will need to do lots of marketing which in many cases can be expensive though if you look around there are a few who will do a free listing and take a commission on every rental. We have done short term rentals with our apartment and normally can fill July and August without much problem though other times of year can be hard, expect about 10 weeks per year. Plus you will need someone you can trust to manage the place locally.
Long term rentals - these days long term rentals can be a good way to go, we get many enquiries for 11 month, 6 month and 3 month rentals, lots want June to October or November to March/April, but again you will need a local person to keep an eye on the place and do minor repairs. Again some sites will do a listing for free and charge a commission.
If you know the family then why use an agent, sorry agents, just use an independant solicitor and make sure everything is legal and all bills are settled as they will not be friends of the family if it goes wrong. You can make offers via your solicitor and they will do all the checks for you.
If you are thinking of buying the place, do like you would at home, go back and have a look around the area, go back in the evening, in the morning and at weekends, are there dogs barking, youngsters on motorbikes and not nice people next door. Do your research, if it takes a few months then why worry, you are in no rush, just make sure it is right for you.
You say about being on a golf course, we are next to a 5* golf course and Spa and in 6 years we have never had any golfers. We are just 10 minutes drive from a blue flag beach, but even that is to far for some people, they want to be front line and that is to far for some, so dont expect to much and then when you do get something, it is a bonus.
Good luck with it.
Dave