the FS90 from stihl is lieght weight, reliable and if you are eco- has the lowest emmisons on all the brands. plus there is a system that you can purchase edgers, chainsaw and pruner attachemts that use all the same engine. for a homeowner it is a really good tool... one other trick, Stihl uses all the same pole diameter... all the parts will go on a weedeater head... but the attachemnts as you need them and you are saving $$$$$$..... it adds up fast on theese tools....
i also recommned the BR600 for a blower... you will need that next! lol.....
On the fence line a you can sprya round up, but you can add a pre-emrgent to the mix to stop the weeds from coming back.. it takes time to get a layer down, but once it is down then you spray once in a while to reinforce the barrier. It is the same method that turf chemical companies use to keep the weeds out of your grass. it does not effect the water table not any vegetables that you might be growing... You can also use it in the beds around the house to keep the beds from growing there as well...
and find a place like a John deer Dealer that you can purchase the round up or similar at... do not buy that junk at home depot, you will pay to much there. You are looking for glyphosate 41% or 38.5 % depending on what stock is left. your mixture rate is an ounce per gallon. I would buy a 2.5 gallon let you last for years!!!