1994 Mazda 323 New Starter Motor and Alternator but engine

After a slight starting problem I took my Mazda 323f 1994 into a garage where the alternator was replaced despite being less then 18 months old. The car was jump started off the forecourt and I was then told to take car home 1 mile away and charge battery which I did. Car did not start following morning so I returned 323f it to garage the starter motor was replaced. Dose the starter motor and alternator often Need to be replaced together? This time car started and I drove away after less than 15 miles I noticed a high engine temp and opened the windows and put the heaters on full to draw heat away from engine. No heat came from the heaters and the cold air was minimal. This is the first time this problem has occurred in 6 years. Trying to get back to the garage the car overheated and coolant boiled back out off expansion tank. The garage now says I need a new engine as there is no compression in the cylinders and the spark plug leads have melted, They say the engine is "cooked". After spending near £500 I now have no money, is there any other cheaper way around this? Any advice you can give me would be appreciated.