Life jackets (one for each person on board and accessible)
VHF (pref fixed DSC not portable)
GPS (to link to vhf as well as navigate)
Compass
Inshore flares
Ladder for getting back on board (sounds daft but you may well need it!)
Torch
First aid kit
Correct number of fire extinguishers (dependent on boat size)
Bailer or bilge pump
Throwable ring for man overboard situations
That's your basics - don't skimp on the cost as you could come to regret it!!
Hope that helps.
Don't forget an anchor! Don't know how big this inflatable is, but if it's around the size of thornbackflounder's then things like a fixed VHF and lifebuoy might be impractical - the lists above are excellent (as is the rest of Salar's site) but I'd say they're more geared to the typical angling boat rather than a 3-4m inflatable - for instance, you're secondary means of propulsion is more likely to be the oars rather than a second outboard.
For a small inflatable I'd be looking at hand held (waterproof and floating type) and perhaps a line with a heavy knot tied on one end for throwing to a man overboard could also be used as a tow line. If you're not going too far from shore you may also find a pack of mini-flares easier to carry. In this size of boat where you can be unexpectedly 'bounced' out, I'd also say lifejackets should be worn at all times not just accessible (your body is also the easiest place to keep them in a small boat!) and they should also be equipped with crotch straps, spray hoods and lights - often sold as extras so you need to budget for this. In a small boat I'd also keep the vhf clipped to the lifejacket and a mobile phone in an aquapack.
A bow dodger bag like the one you can see in the pics of thornback's boat would also be a good idea to carry things like water, warm clothing, the flares, spare kill cord (again essential for a small tiller outboard) etc.
The thing is, safety equipment can vary from boat to boat and situation to situation, that's why the RNLI's SEA Check can benefit you - it can be done before you buy anything. If at the moment you're just budgeting rather than buying I know someone who's just equipped a 3m RIB and I think he said that the equipment came to around £600 - but I think that also included a pair of the large type launching wheels for the transom.