Customise a Wordpress Theme

So I am in the process of building a small wordpress website which is essentially a car review website of sorts, but the primary focus is for customers to submit their own reviews. I have purchased a wordpress theme already, see below: [login to view URL] Currently I have been working with this theme in MAMP, trying to customise it to my needs. So far it is going well, other than the main issue I currently have is that the theme only lets myself submit the reviews, whereas I want the customer to be able to do this. In fact I want more than one customer to be able to submit a review on the same car / post. This should all match the style of the rest of the site btw. So the first thing I would need help with is to install a new plugin or tweak the one the site currently uses in order to have user reviews submitted, I am informed this should be pretty easy for someone who knows what they are doing. Once we have this review issue sorted I would also like the user to be able to sort the reviews by rating, price and location. I am looking for someone who can help me with these sorts of things over the coming months on an ad hoc basis. I will require a more custom site built from scratch further down the line, but need to make this one great for now. I am advised that costs should be between £50-150 if this sounds reasonable? Ideally I am looking for someone UK as based as this will lead to further development work on the site.