Fix and customize already Wordpress site

Currently there is a registration (“reg”) plugin, a reg button and a log in button, when the reg button is clicked, the reg form pops up. There’s also a Mailchimp account where the the reg form has it automatically checked to subscribe to newsletter. There are some problems: - If there is an error when filling out reg form (e.g. password mismatch or field not entered) the form doesn’t notify the user that there is an error, the form closes and it could be perceived as it’s all done and successful. - Upon registration, I am not getting email notifications to my email address. - When someone does register, I want the form to give confirmation to the user that it’s been submitted and also it tells them to check their email to confirm/activate. - When someone logs in, I want a “Welcome (username)!” displayed to replace the buttons (currently the buttons disappear once someone is logged in. - Just like how the reg form is done with a pop up, the same thing can be done for the log in form. I want everything to just work. I want someone to register, the form confirms, they get the email, they activate, it leads them back to the site home page and there’s a “Welcome (username)!” there in the same orange colour as the buttons. If they log in, pop up comes up, they log in, they get confirmation welcome username on the home screen. Please only very good operators/coders apply, I want this job done right. Please give me a quote of how much and how long and also confirm that you can 100% do all of this as I've had a few people tell me they can but then later on say they can't.