1st Gen R53 Cooper S Rebuild in my future

Well today I replaced the belt tensioner and found my oil leak on the backside of the block to have gotten far worse. I am unsure of the motor design so I don't know if the head has oil ways on the pulley side that could leak out onto the timing cover. The entire timing cover is full of oil and grease, and the crank pulley is covered as well as below the exhaust manifold around the filter housing. That was just the visual of whats going on.

The car has been having issues maintaining temperatures at idle(getting up to 205 without cooling down in 30-40 degree weather). I replaced my radiator and radiator hose kit, thermostat, reservoir tank and after adding new coolant to the system and running it i found what i thought was oil in my coolant. The car has 264k miles on it. Then today while topping my oil I found milky fluid on the dipstick. I also feel the car has been losing power. While boost is still fine it seems like the coolant has a lot of pressure inside the system when opening it even at 130 degrees coolant temp it still hissing quite a bit while opening the reservoir. I am looking into options of whether to rebuild my engine or pick up another low mileage around 70k motor.
If say I want to rebuild my engine stock what kind of price would it be to rebuild? I am thinking new head gasket bolts, head gasket, head gaskets, Rod bearings, main bearings, piston rings, would i need new pins and are ours floater style? I also would need front and real main seals, new ati dampener crank pulley. What website could I get oem quality parts from for cheap side? I dont want to spend a ton. I been on mini mania and saw their headgasket set and had a thicker head gasket 0.95mm or .65mm. Would I be best off getting arp head bolts? Would I need to replace connecting rod bolts and so forth? I tried googling and searching for information but i get caught up in a bunch of supercharger rebuilds or partial rebuild kits. Also i am sure I would need a full timing rebuild kit.