Ok, that’s helpful :) A good starting place to look would be something like this
http://www.bestbuy-mobility.co.uk/images/wa-roma-3-walker-chrome.jpg.
the wheels will get you over most kinds of urban terrain and it folds for storage. There are brakes to stop it running away with you and you could get a bag to hang on it although this makes it harder to fold and bulkier to store. It will go into most disabled toilets.
This kind also folds but is bulkier. The advantage is that it has a seat so you can rest and catch your breath if you need to, and there is storage underneath it.
http://www.independentliving.co.uk/uniscan/gpa-ed.jpg
Its essential to try before you buy so either order form somewhere that does free returns, or find a shop in your area....and be tough.....don’t let then talk you into anything unless you are sure. Based on what you have told me, i would definitely go for something with wheels. the ones that you have to lift and place are very tiring to use and only really necessary if the person has got impaired balance or cannot manage an aid with brakes.
there are many different varieties within the two types I have listed...dearer, cheaper, heavier lighter, more sturdy and so on. All will have different pros and cons, including price!! I mean this politely but speaking as a large lady, do make sure that whatever you choose is rated for your weight.
I hope this helps. please do shout if you would like more help.