I am currently using garmin cadence and speed sensors with my elite turbo trainer, I am being advised I should have a power meter too, what should I get?
That’s a really big question to answer w/out more details. Do you ride your bike outside a lot? Is this a road bike or mtb bike? Do you have multiple bikes? Do you like your trainer? Etc.
You could upgrade your trainer to one that has a built-in power meter. You could also go the other way and get a power meter for your bike. Either way, there are many options w/pros and cons. Also the cost can vary substantially.
I ride my road bike out from April to October. I use the same bike on my turbo trainer.
Do you use Look cleats? If so, then perhaps power meter pedals such as the Favero Assiomas?
My question is WHY are you being advised you should have a power meter? A power meter would definitely provide more accurate power information to Zwift, but if you’re just riding Zwift for fun, is that really important? (I’m not saying that’s the only reason you’re riding Zwift, just asking a question.) In the end, you would need to determine the relative importance of having more accurate power data in Zwift (and being able to use the power meter outside, also, since you say you are using the same bike IRL) and whatever else you could do with that money.
If you do decide you want to get a power meter, though, I’ll second the recommendation for the Assioma pedals. I have the Duos and love them. Easy set up and easy to move from bike to bike if you choose.
Be careful, though, the next thing you know you’ll be wanting to get a smart trainer, too! 
If you are getting more serious with your training, you will probably benefit from a power meter on the road. Personally, I think the greatest use of the power meter is that you no longer stare blindly at the speed. Since the conditions change (e.g. wind, uphill), the power gives you a more accurate number to “effort” which will, most importantly, stop you from going to hard on the hills and against the wind
I started zwifting for Scotland so they are are saying I am on Zpower and need a power meter or a smart trainer. Wouldn’t bother upgrading otherwise!
Power pedals are the easiest…Favero, Garmin, SRM, or PowerTap