Unfortunately I can't give you any stats on the difference between using the SaleHoo contact form or not, it's something we don't collect data on to analyze. I can tell you that the reason we introduced it was because we had a lot of complaints from members in regards to suppliers not responding to their inquires.
I would like to be able to say that we no longer get those issues, but unfortunately it seems to come down to the supplier. Some, for whatever reason, just refuse to provide quality customer service.
I would suggest that it certainly can't hurt to use the SaleHoo contact form, but about the only way to guarantee a response to an inquiry is to actually call them. It's very unusual for any supplier to simply ignore a phone inquiry, no matter how busy they are, where as email inquiries are treated as a non-priority by far too many suppliers.
Given that you are registered for tax you will be limited to dealing with either drop-shop suppliers who don't require it or liquidators, it certainly counts out wholesalers and some drop-ship suppliers who do require a tax id.
I don't know what your location is like, but have you considered looking at the second hand market?
Yard sales (garage sales) weekend markets, charity shops and so on, you can be amazed just how much profit you can turn a lazy hundred into if you know what to look for and how to talk a good deal. Yes, it does require more effort than just using a drop-ship supplier to run your business for you, but effort usually equals reward in this game and having been involved in that second hand market myself, I've never seen any drop-shipped products that come close to competing profit wise.
I look at it like this, if someone asked me the question "I have $100 to invest, do I spend that on trying drop-shipping or do I try my luck at some yard sales?", I wouldn't hesitate personally, yard sales is the easy answer for me. So perhaps that might be an avenue worth investigating before doing anything :)