Pressure washer advice
I have a PW with a Honda GPX390 that goes up to 4K, so I know that 3500 psi you mention is really a powerful force. In my opinion, if you go that route, you better cover everything near by - the cast-off spray will be great.
I previously had a Karcher gas powered 1750psi, it was nice, but took much work to do a lot of decking. With the 4K - no sweat, ha!!
Could you rent one for the project? There is a website I visited before buying that showed different projects and what size pressure washer to buy - good advice from it. Let us know how you resolve it.