I have no experience with Designa, or coil springing a Series, but I have both leaf and coil Rovers. I noticed that you are not running parabolic leaf springs. In my experience, the parabolics will get the 109" very close to the level of coil spring comfort, but not quite. If you're currently satisfied with the ride on the leaf springs, staying with that type frame is going to be your easiest bet.
If you're not, well then go for it, but you'll likely need a few more bits and pieces to make it all work (arms, brackets, springs, etc..). Designa can supply it looks like, but that depends on your wallet I suppose $$$.
The last thing to ask yourself, is how long can you afford for the Rover to be down during the frame replacement. Because you can get a 109" leaf frame pretty quick as several companies keep stock stateside. I don't know of anyone stocking a Designa chassis stateside.
61 II 109" Pickup (Restomod, 350 small block, TR4050)
66 IIA 88" Station Wagon (sold)
66 IIA 109" Pickup (Restomod, 5MGE, R380)
67 IIA 109" NADA Wagon (sold)
88, 2.5TD 110 RHD non-hicap pickup
-I used to know everything there was to know about Land Rovers; then I joined the RN Bulletin Board.