We're clearly twins - think of me as the Danny DeVito to your Arnie (I'm shorter, fatter and an apple) 
I don't claim to be terribly stylish but things I've found that help -
Feet; I presume you have orthotics? I've got MN and severe PF in both feet and usually stick to trainers (but a solid black colour). Vans are good. I can't wear Converse because my feet are too wide. I wear DM's in this kind of weather when it's wet.
Hair; snap. Mine is also thick and very fine. I know bobs are supposed to be passe but the shape and length looks better on me than any other style. If you google you can find sites where you can upload a photo and experiment with different haircuts.
Bag; go with whatever is best for your back. Seriously - your 40s are not the time to start fucking about with back pain!! I use a rucksack because a one shoulder bag isn't practical for the weight of my laptop etc. You can get some really nice and stylish looking rucksacks though so practicality doesn't necessarily mean boring.
Clothes; work out your look - and by that I mean your natural style. That's what you actually are, not what you want to be. I really want to look polished and well done, but my natural style is quite relaxed (think smart scruffy!). No matter how hard I try to be 'immaculate' it doesn't work - or I spend the day feeling very uncomfortable.
Working with my natural style made clothes shopping a lot easier - that, and working out what colours suited me. The Kettlewell site has a few good online colour tests you can take. That way when you go shopping you make fewer mistakes. So for me - high necks are a no (big norks, looks matronly), teal is a good colour on me and I need straight leg trousers as they skim and look smart (big calves so slim fit looks odd).
Once you've cracked the style and colours, then where you shop doesn't matter - I'll swing between M&S, Next, GAP, New Look, eBay and H&M.