overheating

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i was recently given a 1995 blazer by a family member to get around until i could afford to replace it. it has a service engine soon light on and the temperature gauge starts to climb within 10 minutes of driving and pegs out with the check engine gauges light on. when driving normal street speeds 40 mph, give or take, the needle will drop immediately when i accelerate the rise again when i let off of the gas. on the highway it may come down some at times upon acceleration but not as much. a week ago this problem was only after 10 minutes or so of running then after accelerating a time or two the temperature would fall and remain normal there appears to be oil in the coolant when i look in the overflow resivoir (it's brownish in color and slick to the touch. it has only bolied over coming out the hose from the overflow resivoir once. i have made sure that the radiator is full of coolant. i've heard opinions ranging from a bad thermostat to blown head gaskets to a cracked block. i don't want to continue to drive it like this but don't want to nickle and dime myself to death replacing thermostat and coolant and flushing, etc if it needs more major repairs.