Why are you buying such an old CPU? It’s not bad but there are probably better alternatives if you are looking for something right now.

If you want to minimize the risk of XMP not working get the 6000 kit. Since it’s cl30 it probably has hynix chips anyway, which means that if you ever want to overclock in the future you’ll still be able to reach high frequencies.

The u12a isn’t a top tier cooler but it’s also true that if you mostly game the CPU will see relatively low usage. And with undervolting you can gain a lot in efficiency without sacrificing too much performance. All core workloads could suffer a bit more though.

If you think it’s the right CPU for you I’d say go for it and don’t worry too much.

There’s no need to stick to an arbitrary number like 65w, change it at your will depending on your cooling or desired power consumption and the CPU will do the best it can with the given power budget.

I doubt the bios of an hp prebuilt has any useful option but on normal motherboards there’s a tab to set cpu max boost depending on amount of cores active. Check if it’s limited to 4ghz, and try to play around with power limiting/saving options if there are any.

If it doesn’t work you could try using throttlestop or intel xtu.

Should work fine, but if it protrudes too much it might keep the cooler from having a proper contact. You could sand it down if that’s the case.