Yeah, beller gives good advice... Wait to get the system before purchasing anything additional. The A4000 can output a signal compatible with a standard VGA monitor with no additional hardware.
The problem is, it can only do that in Workbench and compatible applications. (Typically not games, nothing you have to autoboot into, etc...) But, those screens can be displayed on a TV, if need be.
So, you'll have a way to test the system in the configuration it arrives, and then can decide from there if the purchase of a scan doubler is necessary for what you'd like to do with it.
Honestly, if you're planning on selling on the Toaster and installing 3.9... You'd probably want to look into a Cybervision 64 (or CV3D with attached scan doubler option) or a Picasso IV card, instead of just the scan doubler...