Hi and welcome

As far as the misting up goes, there are two things you should do. Firstly, clean out the scuttle drain following the guide on here in the maintenance section (let us know if you can't find it). Secondly, replace your cabin filter, which is located juet below the windscreen on the passenger side, under the bonnet. This should cure it. Driving with cilmate in "Eco" mode (if you have that on yours) will cause misting at this time of year also.
When doing the timing belt change, I must stress, you need to change the whole timing belt KIT, which includes new tensioner, new guide pulleys and new bolts. It's the tensioner that fails before the belt, which causes the timing to slip and bend valves at best. Needs to be done every 40k miles, and needs to be done now if you've no solid documented proof of the last change. Changing just the belt and not the kit is crazy.
Use Genuine Vauxhall engine oil is my advice, others may vary.
Fitting cruise on yours is a peice of cake. All you need is the stalk, which is a 5 minute job to fit, and to make sure you have the correct pedal switch, (the one Tunnie described in your welcome post). If this is the case, then the only other thing you need to do, is get cruise enabled if it is not already. You need someone who has a Tech2 for this, Only Vauxhall dealers will really have it, but if you're nearby then TheBoy offers a very reasonably priced tech2 service.
As far as turning off the fuel computer etc, I'm not aware of a way to turn it off. You could do something really crude like unplug it, but I would strongly advise against. The computer is a little gem for alerting you to problems before they become serious, such as oil level, coolant level, brake pads, even washer fluids and lights not working.
Hope this helps, Enjoy the Omega, they are a great car.