Are all wheel drive cars suitable for F-roads in iceland?
The rental car agencies decide what cars they allow on F-roads, so you would have to check on their websites, but in general it will have to be a 4x4.
When are you planning to travel and what routes are you planning to drive?
Most F-roads will require a large 4x4 with higher ground clearance, not a city SUV and will require experience driving on rough mountain tracks and fording rivers with a vehicle. Most F-roads are open between July and some time in September, but opening and closing dates will vary every year. Driving in the Highlands requires not only an appropriate vehicle, but also an appropriately skilled and experienced driver to avoid expensive damage and accidents.
The plan is for a summer visit and mainly concerns the final stretch to Aldeyjarfoss and in case one is caught on an F road
You can spare yourself the extra cost of a 4x4 in this case. It's only 3.7 km, and it would be a lovely 30 minute walk ... or hook a brief ride with some other travelers, if you see some who's going there right at the same time.