Welcome to the forum.
Here is an online service manual that is very helpful.
http://skinnerbox.steaky.org/123_DISK2/program/123-cd-index.html
The redline of the car does not really matter. The injection pump will not let you overrev the engine.
The 1,2 and 3 yellow dots on the speedometer are the maximum speeds for that gear. so if you want to manually downshift don't do it if above the max speed for the gear. Actually the tranny won't let you do it.
Your first Mercedes 300D. YOur car may be operating and driving normally. These are a different beast to dirve than any American car you have driven. First your turbo gate does not open until 2000 RPM. So you will notice this boost at 2000 RPM. MAKE the car shift to get the engine in the 2000+ range. You pedal has a downshift switch. Satrting at a light I floor the pedal. As the rpm's incresase I may let up opn the pedal but not much. This will be the first car that you dirve where you floor it all the time. If at speed and you want to accelerate, press the pedal to the floor and force the "downshift swithc" to downshift the tranny. Dont' be afraid to drive the engine hard, they like it. As I said you want to keep the enigine in the 2000-4000 rpm range for best acceleration.
If you wonder if it's driving normal. Take it to a dealer and ask them to drive it, they won't charge for this and they can tell if it is fine. Or join the MBCA (Mercedes Benz Club of America) go to a meeting and ask one of the members to drive the car.
My recomendations.
Change the oil and filter every 3000 miles. Use the Mercedes, Hengst or Mann filter. Natural or synthetic, your coice.
Adjust valves once a year. Twice is better.
Use a algeacide in the fuel 2 times a year.
Check the vacuum system completely all connectors and actualtors. Review the systems in the online manual, buy a hand pumped vacuum guage. Vacuum controls the following
door locks, brake booster, tranny shift, EGR operation, engine shutoff, injection fuel control.
Keep searcing this forum and the manual, then ask questions, 99% of the questions have been discussed so a search is the first step.
Check your flex disks, Item 41 propeller shaft. If the fron one breaks it could total the car.
Blowing the fuse is not right. Mercedes usuall don't blow fuses. You need to fix this.
Converting to R134A Unless you flushed the system and changed the seals to R134a seals your setting yourself up for a system failure. the R12 oil must be removed. The R134a oil and R12 oils dont' mix so you dont' get correct compressor lubrication which leads to compressor failure. Do a search on this one. DON"T go withtth efreeze 12 or andother freon but R12 or R134a. You will have a problem finding a shop to work on the system if you use another freon.
Dave