It's up to you but picking a good PSU can help with a smooth upgrade and avoid big headache.  I suggest to get name brands like Seasonic Focus .  Other quality name is SilverStone or the new ATX 3.0 As for your questions: 1.  The suggested ATX PSU are modular, which means you will connect only the needed cables.  The included instruction will tell you which cable to use but yes, the new PSU will have dedicated cable(s) for your RTX 2060.  Often called PCI-E or PEG.  You may use the linked PSU to learn more from description. 2.  Yes, you can upgrade to an i7-7700k, however, the motherboard does not have BIOS feature for overclocking.  3.  The main upgrade that requires most power is graphics card.  A 320w RTX 3080 will need 700w PSU and you are ready for it.  For bonus upgrade, your machine is thunderbolt ready (with header).  For storage upgrade, beside two hard drive cage on bottom, your case also has 5.25 drive cage on top.  It can be used for multi-drives dock with backplane. Don't know which area markets you can access to.  Links to Amazon are provided for referencing only.
I am currently running a Dell T3620 with a i5-6500, Standard Dell PSU 290 watts, GPU GeForce RTX 2060 S and 32 gig of Ram. I use this for X-plane 12 flight simulator.  Every now and then the frame will freeze and x-plane will crash.  Especially on long flights. I checked and the GPU I have recommends at least 500 watts.  My system only has 290 watts. I want to upgrade to the i7-7700k and find a suitable PSU with greater wattage.  The Dell T3620 mother board does not support the usual 24 pin connector so this limits what PSU's I can use.   Question 1:  Is there a PSU made that would correctly support the GPU with a 500+ watt PSU that I can use with my Dell mother board and system. Question 2:  As an alternative, Dell makes a 365 watt PSU that was sold with some of the T3620 units.  This PSU has 3 connectors, my current PSU only has 2.  Is this PSU useable on my T3620, and if so where does the extra connector connect?  
Dell Precision T3620 CPU and PSU upgrade Questions