You won't be building belays from ice screws on 99% of Scottish routes. Only on particular routes (eg on Beinn Udlaidh, Creag Meagaidh, Liathach) will you get that much ice. Rock belays remain the norm for most Scottish winter routes and are often significantly better than ice or snow belays, which should usually be treated with a fair bit of suspicion. I've never seen anyone with a snow stake in winter, and very very few carrying deadmen.
Going back to the question, I'd say you might want to consider getting an ice screw for the Cobbler's gully. I don't think you'll use it (it's likely snow ice and therefore no good for screws) but by buying one you mean you want to use it in future and therefore get more done. You definitely do not need an ice screw for Ledge Route.
For screws, buy a 16 cm one as this will do anything except Abalakovs and I'd get one with a spinny-handle (eg BD Express) as once you're on steep ice they make a huge difference. You might not be on steep ice on grade II but if you're keen to do harder routes in the future it will make a big difference. Even something like Good Friday Climb or Tower Scoop (both III) have a lot of ice on them and decent screws help a lot.
It's a feature that only the expensive options (Suunto Ambit, Garmin's top models) will do. I think they realise this and so charge top dollar for it. It is indeed a pity. Best bet is to find a Suunto Ambit on offer, as they've now got the Ambit 2 and 3 out.
I've climbed Ledge Route but haven't climbed Chockstone Gully. Having had a look at some photos of that route I'd carry a fairly standard multipitch rack (wires, hexes, lots of slings) and definitely 5-10 m of ab tat on it. I would use both your axes, definitely - it makes it easier and does no harm. I'd use the same kit for Ledge Route if you're starting out but in good conditions you won't need much of it (when we climbed it we placed no gear and moved together the whole way). For other grade II routes (eg. Right Twin on Aonach Mor) you might need an ice screw or two to protect sections. However, most lower-grade routes don't have a lot of ice on them.
However, as Pete says, if this is your first time out in winter then there's a lot of things to learn about and having a good grasp of navigation (the summit of Ben Nevis in a whiteout is absolutely lethal if you can't rely on map/compass skills) and movement on snow, and knowing about weather/avalanches/keeping warm etc. is vital. It's often harder than the climbing.
Regarding 'it's only grade II' opinions - grade IIs vary hugely in difficulty depending on conditions and the sort of route it is. I've soloed grade IIs quite happily sometimes because it was the best way up it, and sometimes because it was the only way as they've been unprotectable. At other times, grade II has meant vertical powdery death and has been absolutely horrendous. I've led the equivalent of Scottish V in Europe a couple of times and on both occasions, in good conditions, it felt much easier than some grade IIs I've done in Scotland in bad conditions. Be prepared to back off a lot of routes (probably 2 out of 3) until you've got your experience up. Even then you might back off more than 1/3 of routes. Winter's the best thing about the hills, but also potentially the most serious.
Lots of good stuff here. Though it's a fairly basic idea, replacing stuff that requires boiling (eg pasta/rice) with stuff that can just stew in hot water (cous cous, noodles, polenta) saves loads of gas.
Not drinking hot drinks saves an enormous amount of fuel. I just did a 9-day trip in the Alps (air temperature below 5 °C when we cooked most nights) on a single 230 g gas canister, cooking a meal from scratch every night frying veg and meat, then boiling pasta or bulgar wheat, for two people. That neither of us really drink hot drinks means we don't have to worry about mugs, extra fuel, wasting time boiling water, or carrying sachets of calorie-empty food.