plastic blades won't help with obstacles I doubt, no lawn guy uses them. Just try not the go so deep into things like that and it'll just snip the end of the line and can bump out more instead of it breaking off flush with the head and having to take it apart.
I love 4 stroke stuff because it's cleaner and has a lot of torque, I have a 35cc 4 stroke Honda and it's a beast, it is heavy though.
It's not quite as powerful as the biggest 2 stroke trimmers as to be expected when comapring 2 to 4 cycle small engines and size and weight, and but does have an advantage in torque maybe useful if putting a brush cutter head on it but besides that it's still up there in power with the strongest 2 strokes I think.
I have the 25cc version honda 4 stroke engine in a husqvarna trimmer (all 4 stroke husqvarnas use Honda engines) that I use all the time and it's great. These 4 strokes have oil slinger patent that can be ran upside down for edging and they don't burn any oil unlike the cheaper ryobi etc 4 stroke trimmers I had put heavy use on before.
You can attach brush cutter heads to either of these 4 strokes.
For trimmer line I have tried all fancy twisted braided 'best ever' this and that, but found nothing works better than rino tuff I get from home depot, .095 or .105 it's basically diamond cross section.
Get a Speed Feed 450 (or 400) head, the Chinese ones on ebay I can vouch for are as good as the genuine echo and they are much cheaper and get a couple spare caps made to wear down, but come to think of it I have hundreds of hours on just one bump cap compared to the other types of bump heads I was replacing caps all the time and never bumped either on pavement. Speed Feed is soooo easy to load and doesn't jam often, and if snips off flush (that's what I was saying before about not going to close to obstacles but sometimes edging a sidewalk really deep or something it'll snip off flush, anyway it's not too hard to fix that just open it and unwind just a few inches to make it out the eye again and don't have to unwind the whole thing like some other bump heads.