I'll get lynched for suggesting this because I get lynched wherever I suggest it, but I bought a Zomax off ebay for £35 2nd hand, they are only £95 new. I have abused it terribly and it's still going strong after who knows how many days and months of heavy use, it's got all the features of the expensive ones and spares are cheap - including replacement engines, it has 18 inch bar which is plenty big enough for me. And thats with veg oil instead of chain oil (cue another lynching).
You'll quickly end up spending more on chains, bars and sprockets than you did on the original saw. If mine ever blows up I'll just buy another for £95. Mind you, I'm not felling monster trees 8 hours a day every day. A foot at the base is what I'm mostly felling.
For a brush cutter I have a long handled Ryobi petrol strimmer with a brush cutter attachment blade, wicked thing but works on anything from heather, stunted trees to bracken. You can also get a mini chainsaw attachment for it which is VERY handy indeed for getting into gorse and dense lower branches. Non ryobi attachements fit it as well. The strimmer heads not very good though, so I use the brush cutter blade for everything.