120 or 240 sling is grand, preferably both, if you can spare the cash. I would tend to recommend a 120 to start with. 240’s are mostly only useful in build anchors and gear nests.
The thinner the cord, the more the prussik will ‘bite’, so it depends on how what you prefer when abseiling. I went for 6mm, cos that’s pretty much the standard. 8mm might work, but it might not bite enough, depending on the slickness of the cord. I’d say 1.5m is a bit long; 1.2m or 1.3m is an easier length to deal with. I just ordered 4m of cord and got them to cut it into 1.2m, 1.3m and 1.5m lengths. I’ve yet to use the 1.5m. (If you do order a length like this, get them to cut it, cos they’ll seal the ends, so it doesn’t unravel halfway down a cliff.)
As many carabiners as you can. One for each sling, and one spare, as a minimum, I’d’ve said. I’d also recommend a snapgate or something to haul your walking boots up on your harness, so you don’t have to walk off in you climbing shoes.