How to find the right SEO consultant?

As a freelance consultant myself, I'll give you my 2 cents on the information you've given.

You say your goal is to rank 1-3 on Google for certain keywords. Is this for general business exposure or to gain leads/get people to actually come along to your yoga classes?

I ask this because your workshop will be set in a certain location in which local SEO would be highly beneficial over simply higher rankings for locations all over Germany. A lot of people say "I want to rank high on Google" but you should really think about why. What are your true business goals?
Anything that is done digitally should align with those goals, it also helps to map out much more effective on-going strategies.

In terms of payment, it depends on what both you and the freelancer prefer and agree on. You will see some freelancers that require hourly rate as mandatory and others are more flexible, eg. payment for certain 'services' once complete. It gets tricky with SEO because sure there are step by step processes in optimisation, but it a tortoise vs. hare story. (SEO being the tortoise, paid advertising such as FB Ads and PPC ads being the hare.)

Other things come into play as well, such as the design of your website. Someone can rank you on Google as you wish, but your website may not be designed appropriately, which will equal high bounce rate and low conversions. There are so many things that come into account for a job like this, picture many little cogs working together to make the entire clock tick.

Don't get me wrong, if your goal is simply have more people sign up to your workshop, exposure, or whatever else - it's entirely possible and doable!

With a constrained budget your consultant should be able to talk with you about the available options for the budget - and you both and discuss and perform those that are the priority and most beneficial for your business goals.