If you are just doing indoor trainer rides a power meter is, IMO, not necessary. As much as bkool gets roasted for inaccurate power, it has, IME, been consistant in what it states. I would expect most other trainers to be similar in their readings, consistantly inaccurate to a greater or lesser degree.
Yes accurate numbers help in planning your training, but are not a requirement, consistancy is.
@Whorty you need to decide what type you will be going for. Buying anything but maybe a pedal meter may require additional kit, iirc you are using spd mtb pedals, so a pedal system (all seem to be the look keo/spd-sl type currently) would mean new shoes as well. Speak to your lbs about compatability for your bike and make your choice from there.