This is exactly what I've been doing for the past few weeks!!
Like you I wanted the creams for other people to have a bit of a shelf life. After a bit of researching preservatives, I've found that;
a) http://www.aromantic.co.uk/index.htm sell preservatives for creams - Preservative 12 they tell me is milder than Preservative K, although the latter is recognised by the Soil Association for use in Organic products. 12 is good for creams, but can not be used in products with detergents, like shower gel.
b) Lots of organic and natural cosmetics companies are using Grapefruit Seed Extract as a preservative as it is antiviral and antibiotic, and non-irritant to a dilution of 2%. I've been taking it as an anti-candida supplement, so I had some already in the fridge
c) Adding Essential Oils automatically increases the shelf-life, as most are anti-viral and antibiotic to varying degrees.
d) Adding an antioxidant is quite easy too - Vitamin E does the job, from a supplier, just open some capsules up, or add another oil rich in it - like wheatgerm oil.
I'm going for the Grapefruit Seed Extract and Wheatgerm Oil options - which should increase the shelf-life to a month or so, especially when refrigerated.
I've found it really hard to get the mixture to emulsify properly - the web is peppered with advice for how to get it to work, so it's obviously a bit tricky. I do need to get a thermometer I think - and I might try adding soy lecithin to help emulsify the mixture...if I can find some!
I'll be interested to hear how you get on - please offer advice back if you get the lovely whipped cream texture I was hoping for!!