We have both Black & Decker electric and Stihl petrol. The latter is better for heavier work but the weight of the engine plus fuel is that much heavier. My OH uses the electric one quite happily. As for disposal of the clippings, as Lizzie27 says there'll be a lot of them - but they burn beautifully!
PS - Have you consulted the owner of the other side of the hedge? The option I overlooked was to engage a man with a chainsaw, leaving you to do the disposal but I shouldn't attempt hiring one yourself. Chainsaws don't take prisoners.
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Rather than use a long reach I use a decent tripod ladder but that’s likely to be even more money!
The other option would be as others have said to look at getting someone in.
The only other route may be to try and hire a more ‘professional’ hedge trimmer at least that way you may get some idea of your need?