The venue needs to be accessible to anyone who wants to attend. Try to find the most accessible venue you can, as you can't predict everybody’s needs. It's ok to ask people who are planning to come if they have any specific needs before your meeting. Find somewhere with good public transport links and reasonably priced or free parking. At your first meeting give people the opportunity to feedback on the venue.
Keeping costs down
It might be cheaper and easier to meet in a public place, like a coffee shop, but remember that for some people these venues can be too noisy/busy. It might be worth finding out whether there are quieter times of day at the venue. Think about the purpose and aims of your group and how people might feel discussing these themes and issues in a public setting.
There are other venues that may offer a quieter space while keeping the cost low. For example fire stations and some supermarkets offer community rooms which you can use for free. Your local CVS (Council for Voluntary Services) may also have some useful advice as they help community groups to grow and develop. You can search on the internet for these services and other free local community rooms.
Champions' daily lives are different: some people may only be able to do evening meetings, whereas others might have spare time in the morning. You want as many interested people as possible to be an active part of the group so if you can't find something that works for everyone, you could offer alternate daytime and evening meetings to give people most opportunity to attend.
How often and how long you meet for depends very much on what you hope to achieve and what deadlines you have in mind. It is important to consider how much time people can give, and what can you can achieve both within and outside of your meetings.