The number of lectures depends on the university. To be honest, the distribution of the lectures is probably the biggest barrier - One lecture in the morning and one in the afternoon means finding a job would be difficult because lectures cross over those two key times. The university usually employees its own students for some roles, and local businesses can be quite relaxed too - We have a Costa Coffee inside one of our buildings, and they seem to allow students to leave for lectures during their shift, or heavily base it around that sort of schedule.
18-20 hours a week of lectures, tutorials and labs probably isn't a bad estimate. The worst I've known is for some students to get 22-24 hours a week as we do some half-modules and due to timetabling, they had both half-modules in the same term. Many universities give you a day or afternoon off as an opportunity to do sports, although you don't have to. I usually get Wednesdays completely free (although this term I do not), so there is that. Just remember that your timetable may change termly, monthly or weekly depending on what work needs to be done, so during 9am-3pm is usually a bad time for a job. 3pm-6pm can also be risky and I've had a few lectures go on that late, but it's not too common. When we had projects to do, I would be in the workshop for 5-10 hours a week, so anticipate the effort you'll have to put in outside of what's scheduled for you.
Weekend work will be your best bet. I'm a second year mechanical and to be honest, I pretty much do nothing at weekends. I do work when it's required, but I'm usually quite free.
In terms of workload, it really depends. I was able to do little in my first year and still get some great marks. My lectures were usually over by 3pm and all my work was typically finished before 6pm so I had all my evenings off (until around the time of deadlines/exams where you up the effort). So it really depends how much work you need to put in, but I didn't find it as crazily busy as many people suggest it is. It's just tiring though with quite a lot of contact time, so I like to keep my weekends clear for sleeping and relaxing.
If you want to get a job, I would consider leaving it until after Christmas just so you can fully appreciate the workload you would have to do. Build up commitments instead of starting with many and having to drop them because that would just look bad. See what your timetable is like and see if the university is hiring in one of their receptions, food stalls or wherever else - They should have the most flexibility regarding employment. Just expect your timetable to change though. I've had to miss a few important sessions because I had booked one thing and then they changed my timetable and added a lab when I was not going to be there. Weekend work would probably be the most reliable for you but perhaps the trickier to find as everyone else will be looking for it too.