I haven't shot that particular Echo1, but my primary is an Echo1 ASR. Echo1 build quality over all is good. The ASR shot a solid 450 out of the box and internally is a blend of the APS2 and the L96, so parts are easy to find-ish (depends where you are and if you mind waiting for internet shipping if your local shop doesn't have parts for that particular gun) The reason people like the VSR-10 is because not only are there a lot of parts, but any respectable shop will cary parts. I don't know what the M28 is based off of, but if you really want to go to 550 and you have a local shop, I would recommend looking at the parts they have in stock regularly, just to make it easier on yourself. Any gun is capable of being upgraded, and ALL guns will need to be upgraded for the accuracy. So you will be going into your gun and breaking it down all the way for one reason or another. So unless price is an issue I would recommend getting something with a good body, and get the parts to upgrade as you need them. You can save money getting something that shoots 500 out of the box, you could probably even squeeze 550 out of it with the stock parts, but you'll still be tearing it down to tune it and get the accuracy you want. Make sense? The other thing is what I mentioned earlier, if you want 550 out of a springer, don't expect your bolt to last too long.
One other thing to consider is the BB weight. Typically when someone says "I want XXXfps" they are referring to the FPS with a 0.2g BB since that is the standard chrono weight for fields. If you are looking for 550 with .3s or even .4s you will need a very powerful spring and hence, getting back to the bolt, you will destroy your bolt faster with a spring gun.