Haha, you're in exactly the same position as I was. We spotted a betta lifespace tank for £300, then other second hand bargains, or had the option of buying a tank and starting from scratch. Now, the general opinion given to us that the smaller the tank, the harder it is to keep levels stable. So strangely, with marine life, the bigger the easier!
But, we really don't have the space for another large tank(however, we now think this one is too small LOL), so opted for the 10 gallon(tank, btw, is aqua trend 418). Anyway, I spent a few weeks researching everything thoroughly, knowing the size we were going for which was between 10 and 20 gallons. We looked at the Boyu's quite a lot, but soon learned that a lot of modifications had been made by people who had bought the "plug & play" set ups. They did the job, just not as well as they could have if that makes sense? So by the time you take into consideration spending £2-300 on a supposed plug & play tank, then another couple of hundred(possibly!) on mods for it, we were well decided on just putting everything together ourselves.
Take you're time over it and it won't cost you as much as you think. The tank cost me £50 itself(t5 lighting & with the stand), the skimmer was brand new in the box from someone on UR for £20(somewhere around £60 to buy off the shelf), heater I have loads of spares so wasn't a problem, Moonlights were £13 from fleabay, Powerhead(Tunze nanostream) was £20 barely used. Then add Live Rock @ £83 altogether, and coral sand was £20 because I wanted the smallest size, so it had to be the biggest bag LOL(don't buy live, it'll soon become live itself.. just another gimmick to make money!). So in total I've only spent £206 for everything. I could do with adding another powerhead, a smaller one and will be doing that in the next few weeks but for now it's just sitting happily
Forgot, I was £42 on the test kit so £248