So which road should I take?
1) stick with buying a mediator
2) buy a bvison card
You'll have to decide that on your own ;-)
A mediator has three advantages over the BVision:
1. you can use a standard sound card with it
2. you can use a tv card with it
3. it's slightly faster/better for 3D games
If you need a sound card (and don't have one connected to the clockport already), get the Mediator.
If you need a tv card, get a Mediator.
If you're really into 3D gaming, get a Mediator.
Otherwise, get a BVision, it's a whole lot cheaper (eBay) and you don't need special cases.
Im also wanting to buy a scan doubler/ff will I need an internal or external version to be used with a graphics card.
Neither the BVision nor a PCI gfx card plugged into a Mediator support this (only some older, Amiga specific gfx card do). You will always have to use two monitors, unless you are willing to buy a tv card and connect the Amiga's native display to the tv card.