Hi Sharpink,
As you have a limited budget I would personally get a company to create some email templates for you that are tested across all platforms and devices. I had exactly this problem, I didn’t realise what a difference it makes until someone showed me one of my emails on their iPhone and to be honest I would have deleted it if I had received it, so definitely either find a template or two you can re-use that works for all or use a specialist who can help create what you are after, I used <Self Promo URL Removed> who I found were excellent, it may be an extra cost but as you pointed out it will be cheaper in the long run.
It is also worthwhile thinking about your subject title as generally speaking most mobile devices or mobile versions of the email accounts won’t display more than approximately 65 characters, so you message could get missed or cut short.
I started writing my subject tiles to include words like Amazing, Don’t Miss This, The 10 Best… which seems to have attracted more views and converters to the site as well.