Hi Tanya, SEO has become much less of a game than it used to be, and pretty much anyone can do it these days. Provide your customers with unique, well-written copy and you have done most of the job. Using a site like moz.com you can easily do the keyword and competition analysis yourself, so I'm really not sure what someone would be working 8 hours a day on. Building backlinks is the trickiest part of the process, and that is usually best done by you as well - posting in forums, guest posts in blogs, getting your link on trade web sites, etc.
If you do decide to go with an SEO firm, you should speak to two or three references who can provide real feedback on exactly what this firm has done for them. You should also get a breakdown of exactly what they will be doing for you as well. Does it include copywriting? Keyword research? Link building? Adwords/FB advertising management?
The danger of hiring an unknown firm, particularly from overseas is that they will use 'black hat' techniques which will provide quick results, but will get you banned in Google. Based on what you have said above about this firm, I would be very cautious.