I know this is an old post but might be helpful to someone else reading it.
I used a curved shaft edger. I am actually using the short pole hedger in a pinch (yup and it works great actually) and a trimmer with an edger blade above a fixed line string head. Remove the strings and you have an edger, put them in and you have a stringer/brush clear master machine. I tend to run with the edger backwards to clean out the line and/or forward just to get a fine edge fast. I make my lines then round them off with the trimmer. It takes more time to beat out the line with a trimmer alone and will not deal with SA runners very well. It can be done. I plan to try the edgit with a 30cc Maruyama, still shopping. That may be a good all in oner. I get massive accounts because of my edges. OF COURSE they want the same price, but there you go, I still win, I still average 30-50 per hour and I just enjoy doing it anyhow. My Shindiawa edger finally died. As for putting a metal blade on a 45 degree head, BEWARE, it takes practice and a little handle adjustments to work. It will travel and it will cause serious damage to edges and shrubs, not for the faint of heart. It works for now. I am in the market for a new edger, but can get by without one. Get a light weight edger, I am looking at the Kawasaki combo now, and always trace all beds and hard surfaces. Makes trimming a breeze if needed at all. If you use something like a 30" toro and edge at a slight back angle to dress the top edge of the grass 'edge' your trimmer will be better used for cleaning beds and touch ups. You can not get to all edges with a trimmer without hitting plants. I suggest you make a new line far away from any sprinkler heads if possible and go ahead and clean out any roots by hand or at least make sure its a root you are bashing into.