Its far from it, even for me. Its a shed load of work, but in my honest opinion its your best way forward. It'll also take time to do, and you'll find a fair bit of the cars dash will need to be removed as well (ideal time to do the climate control mod if you've been considering it!!) but if you plan on keeping the car, its worth every second of it.
If you change the engine full stop, whether it be to LET or X25XE, you've got a lot of work ahead. The V6 is easier in a sense though, because your not modifying the loom, your simply replacing parts like for like. You'll end up with an installation that was meant for the Vectra, and you can easily mod the V6 to around 200-220bhp without serious expenditure. Its a lot of power for the money.
P.S. If you want the climate mod, then get a CDX V6 manual with climate control, and while you have the dash out fit the climate housing at the same time. You can literally just plug and play then! Or if not, get a GSi thats been rear ended or something.