The
DHS660 is a better bet for use on guide rails, partly because it has a better quality
guide rail adaptor (metal as opposed to the plastic one the DHS680 has which is prone to warping, or arrives conveniently pre-warped - had two of them like that before I gave up). I also think thet a right blade like the DHS660 will be easier to use on rails than a "lefty" like the DHS680
Frankly I think you'd be better off saving the money (on the guide rail and adaptor) and making yourself a saw guide from MDF and softwood instead, at least for a one-off. This is the sort of thing I'm talking about:
There are fancier ones, like this one
but a basic guide will do good service (used one myself for years). The other thing you'll need is some form of cutting table to break your material down on top of. This is a rather nifty idea which folds away:
although I'd probably make mine a bit smaller. On site I generally make-up a table which is 7 x 3ft (2100 x 900mm) because that is big enough to support an 8 x 4ft sheet - and I'd go for 2 x 2 CLS rather than ripping down 4 x 2s for my timber. My cutting tables are also a bit simpler - they are just screwed together without any fancy pivoting joints, but they donm't need to get stored as they go into the skip at the job's end (nearly 2 years for the last one). Make sure that you sink any screws, etc about 10mm below the top surface so that you won't wreck a blade on them. You could equally get away with three of four straight 3 x 2 CLS pieces laid across two trestles for a one-off job, but that said it's a nifty idea which I may copy (and modify a bit) for home myself
BTW there's nothing wrong with the DHS680 - I use one regularly as my "go to" saw and it's a really good little saw which I wouldn't do without.
Another little recommend - if you need blades for the DHS680, switch to DW Extreme blades - in my experience they are better than the Mak blades (more durable, less likely to loose teeth if you hit metal, cut smoother, etc). SFX do a set of
three blades for about £40 (2 x 24t, 1 x 40t), but for MDF you only need the 40t blade (DT10301-QZ) and there are people on eBay selling them for around £13 or so if you look around