Oh man I went down that rabbit hole a few years back and switched exclusively to a hammock and ended up spending about $1700. Mind you that was for an amazing hammock, winter and summer underquilt and a bomb-proof winter tarp with all the poles etc. I tried using a foam pad for underneath and it wasn't great. The underquilts are total game changers for sure. My Hammock is from Dream Hammock (amazing company), and my underquilts are from Hammock Gear and UGQ. Tarp is the Simply Light Designs Winter Haven.
My suggestion would be to get a winter rated underquilt. It will cost a tad bit more but for shoulder seasons you will be glad you have it. You can always vent it if you're too warm..but think of it like this: your mattress at home is very insulating and it works just fine in the warmer months too, right? I'd also suggest a wider underquilt as it provides more coverage and doesn't slip out from under the hammock so easily. For a hammock, the longer the hammock the easier it is to lay diagonally for a flatter lie. Look at something in the 11-12 foot length (in my humble opinion).