Welcome to the club. As someone who owns a 76 240D and a 84 300D, I've learnt a few valuable lessons over the years.
My advice would be to follow the recommendations in this
thread to get your mechanicals in optimum condition with minimal spend. Start with Auto-RX in your motor, transmission, differential and PS pump to clean the metal before switching to the recommended fluids.
And despite financial concerns from some, I purchase every filter/ rubber part from the main dealer. Over the past decade, I've yet to experience any failure of a part replaced.