There's nothing wrong with the A55, the EVF has advantages as well as drawbacks.
Fine focussing through the viewfinder on a 600D probably isn't that much better. The 60D does have a larger viewfinder than the 600D though (pentaprism instead of a pentamirror).
It was more like the secondhand accseories like lens that would more available.
What lenses in specific is it you're after? Don't be put off from sony just because there is less choice. Unless you want a specific lens that sony doesn't make, there isn't any disadvantage in the smaller choice. Plus you have old minolta lenses that you can use manual focus, and keep the in body stabilisation (you can use old lenses on canon via adapters, but you don't get stabilisation).
And the lens is more than just an accessory, it's what resolves the image. The lens is what makes the image, the camera body is just something you stick on the back to record it

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The 60D and D7000 might be better than your A55 in some ways, but they are more expensive. For the extra cost of the 60D or D7000, you could get some nicer lenses for your A55, which would give you better IQ than a 60D/D7000 with a kitlens.