I've created plenty of bots previously, the last one I made was used to automatically invoke actions of a character on an online text based game, the game also had captchas to prevent bots from doing exactly what the bot did, but I simply bypassed them.
I've had a look at your requirements, and looked at gumtree's html codes to see if there would be anything to prevent me from successfully automating the actions you wish to have automated, and I don't see any foreseeable problems.
As for the proxy, in this case I don't see the need for one, although I'd be happy to implement the feature to allow you to use one if it is absolutely necessary, there are cheaper ways of getting around the captcha which I'd be happy to discuss with you in private.
If you have any questions, do not hesitate to contact me, despite being new to freelancer, I have extensive experience in vb.net, and can tell you now that gumtree wont be able to tell the difference between a real user and my bot.
Kind regards