Front suspension is not difficult. It's designed to be easy to work on and quick to replace. The struts and strut mounts are reasonably cheap even genuine Volvo - but you probably need to think about buying from overseas. The top mounts are important - I've tried ebay cheapies and "good quality" aftermarket - all junk. Squeak and rattle even when new. Any of the ones on FCPEuro site would be fine - I use Lemforder with no problems.
https://www.fcpeuro.com/products/volvo-control-arm-kit-front-6-piece-xc90-meyle-kit-p2xc90cakt3p6
https://www.fcpeuro.com/products/volvo-strut-kit-front-6-piece-xc90-kit-p2xc90strtkt2p6
This will replace literally every wearing part on the front end apart from the steering rack.
The control arms above include both rubbers which commonly get soft and saggy after 150,000km (Volvo used recycled rubber, which didn't last as long as it should have). The easy and complete method is to buy new control arms complete. Because I have an engineering degree and a hydraulic press, I choose to buy the rubbers and recycle the old arms. It takes a few hours to do the cheap way, so if you pay a mechanic it's much quicker (and therefore cheaper) to just buy the complete arms. Similarly the struts need some fairly beefy spring compressors to change over the springs and install the mounts - please if you haven't done this before leave it to the mechanic. The front springs on the XC90 are quite beefy and you can literally end up with a hole in your head if you get it wrong.
The valve cover leaks are going to be harder. The inlet manifold and injectors need to come off, and that's around 2 hours work for me. The greater problem is that by the time the valve cover seals leak the timing cover (with the similar gasket design) won't be far behind it. Do them both (ie 3 gaskets) at the same time. FCP do a really good kit specifically for this: here's a video which shows how this job is done:
https://www.youtube.com/watch?v=95p8qv8YZ9A
Kit: https://www.fcpeuro.com/products/volvo-timing-cover-seal-kit