Yes you should be medically qualified to inject fillers for a whole host of reasons.
Regarding what type of filler to have - I use all types you mentioned. It is not a case of one type suits all. What needs to be discussed is where the filler is going - for instance I would NEVER put radiesse into a patients lips, anticipated outcomes - are expectations realistic, BIG CONSIDERATION!
Before and after photos should always be taken. Firstly for litigation reasons and secondly the individual has an instant visual (before and after). It is amazing how many people actually 'forget' how deep lines/folds were before. The emphasis should be taken away from filling lines so to speak, but more replacing lost volume and keeping a NATURAL look. The key is for the patient to have a 'well' look rather than being able to tell a filler has been injected. Results should be subtle, that is why it is important to have realistic expectations.
Radiesse - deeper folds (nose to mouth lines and oral commisures), and facial contouring, restylane/juvederm - great for lips.
Botox - upper face (crows feet, frown lines and glabella).
Expect to pay from around £250 up to £350 for one area. Most fillers last around 12 - 18 months (radiesse longer), but again can depend on many factors.
Hope this helps. PM me if you need further info.