Hi Neil,
I think complementing your local seo business for your clients with adwords makes a lot of sense. Since both have there strengths and weaknesses you can maximize the results for you clients while gaining a lot of insights.
Google Adwords express is mainly for small business owners that don´t have the time neither the knowledge to use Google Adwords. You have a lot less control in the former, especially the lack of control over the keywords, and most is automated. This doesn´t give you enough control to effectively manage these campaigns for your clients and add value for them. More info on how to start with adwords for smb you could find here https://www.pickaweb.co.uk/blog/complete-guide-to-google-adwords-for-local-business/
If you should have them all in one account or each one have there own depends on various factors, like:
- Payment - Will each client pay directly to google, in this case you need to create a different account for each
- Recourses - If you want to spend as little time as possible maintaining the accounts this would be much easier in one account with all your clients
- Portability - Do your clients value the fact there account is portable?
- Quality - If you have all clients in one account they all contribute to the overal accounts quality score (positively or negatively) this might not get you the best performance for each client
Hope this helps and good luck