We're about to embark on something similar, but can't afford the floor built up of screed on top joists and frankly don't think 130yr old joists deserve it - so are going down the less optimal (heat sink wise) aluminium plate route. using routed chipboard with pipes and plates sat in grooves. Buuuut, i've read lots of opposing views on efficiency of the aluminium system vs radiators etc which suggest it may not be as efficient.
if you can, go screed - or even some of the products which are castellated trays and you pop SLC on it. if we had the option, i'd do that.
the other stuff to think about is is about how you loop it into your plumbing and what controls it