I think if you search to Volvo website they suggest not to fit rope cutters to the saildrive but I understand some owners do fit them. I haven't heard of anyone fitting a "prop guard" because they would cuase too much drag and water disturbance. 

To mitigate the threat from lines & nets I have a GoPro mount on a long pole that can fairly easily check the sterngear.  Last year I picked up a large fishing net but luckily this was wrapped around the rudder, not the prop. I do have scuba gear but this would be a very last resort in anything but a calm sea (and not with a fouled net!).

I also echo Wayne's advice about the guard. I don't think that the risks of fouling would justify the big hit to performance which in itself might lead to other unforeseen and unfortunate consequences as well as spoilt enjoyment.  

I made my own rope cutter which is a copy of the ambassador. I did have an ambassador but it wore out. I begrudged paying £500 for a new one & have made 2 . One for my old Volvo MD2020 & one for my latest D1 30 . The cost of both, inc materials, Wear in milling cutters, grinding wheels, Pattern for the bearings etc. came to £70 each.

Then  there was the time to make --2.5 days each,  which was really quite fun so I have not put a cost to that.

I have cut numerous pots & a net, (You can both hear it on the engine & feel the jerk on the saildrive, as it tangles then cuts) with the original & my own one. I failed miserably with a sheet of polythene though.

The problem I have is that I have a Brunton Auto prop, rather than a folding prop. So like a fixed prop it tends to hook anything it can. Sailing single handed makes it harder to spot pots, especially at night.

If you do not mind being robbed of your pennies, then I would strongly recommend the Ambassador. However, keep a close eye on the bearings, because if they become worn they wear the cutters & then you cannot replace with new bearings. I got over that by making my own for a while but the blades got very thin as well. In the end I had to replace it.