You need to have this looked at, you cannot simply guess or expect someone to guess for you. There are a lot of reasons why it might not be working (bad wiring, switch, sensor, control module etc.), and it's best to have it diagnosed, it will be a better use of your cash.
The solenoid is part of the starter, I'm not sure you can replace it separately. You will need to remove the entire starter either way.
This switch is very easy to replace, takes 2 minutes. But, there was a recall on these switched, so it may be free! http://repairpal.com/volvo-v70-recall-04v504000 Call your local Volvo Dealer, Smythe? http://repairpal.com/smythe-volvo You may get it for free! Good Luck!
Start by having a shop check that the software is all up to date. I suggest going to a Volvo dealer and upgrading all the software in the car, they often have a fix for this and many other driveability issues.
You are taking an approach to fixing this problem that is all backwards. You need to perform a diagnosis first, then replace parts. Start with reading the codes, then troubleshoot accordingly. BTW, you can check your codes for free, without a scan tool. http://www.troublecodes.net/Volvo/ Post the code here and maybe we can help.
You can get repair manuals online for most vehicles, I would try that. I've done this before but I don't trust my memory to provide a step by step.
What is the oil pressure?
You don't need to remove the dash to replace the relay. If you need to remove the dash, absolutely get a repair manual first. Try one of hese online: http://bit.ly/mitchell_repair_manuals_online http://bit.ly/AllData_Repair_Manuals_Online
This is your oil pump, and you can reseal it. The parts are inexpensive but the labor is not, you are looking at a 3-4 hour job, and you will want to replace the timing belt too, since it will be off durin this repair.
You have two hoses that attach to the firewall, these go to your heater, and it's likely where the leak is coming from. Best to have a VOlov tech address these, the hoses and especially the connectors are real finicky and if not replaced properly, will continue to leak.