You asked about the best solo advertising sellers in Udimi. The niche you are in will often determine which providers you use. The best way to find a provider is to look through the list of sellers. You will see on the right your comments from the buyers. Positive comments are green. The negative comments are red. You can consult them to get an idea of whether that provider will be a good option for you.
You will see the prices charged by visitor to the right as well.
You will have to research the providers that interest you to make sure they provide successful solos.
You can open a free account and start researching. If you want me to give you advice on a particular provider, I will be happy to take a look at your specifications and let you know my opinion about whether I would try that provider.
You will have the option to use your own copy or let the provider provide a copy based on your offer. One thing to keep in mind is that each provider knows the people on their list and that's why they know how to write successful marketing messages for those people.
Providers deliver the number of clicks they buy and normally deliver in that number. But keep in mind that there is no guarantee that you will get the benefits of those clicks. That always depends on the copy.
I hope this helps, and I wish you all success.