That all depends. Is it a G320 with the M104 inline six or the new V6? '98 was the year it chanaged and there are some of each out there.
On the inline six there is a well known tendancy for the headgasket to seep oil on the right rear of the engine. You must look from the bottom. Once fixed, about $2,000 at the dealer, it is not a problem again. Looks like you have a couple M104s already, have they been done?
I don't know about the V6, I think they are pretty solid.
The rest of the truck is tougher than nails. One major service you will want to consider is a front axle repack. Mine is at Europa right now for this. They will be replacing the front CV boots and wheel bearings. My truck has about 77k miles on it now. I had my previous 300GE done at about the same milage. Europa reccomends it every 40k miles. I personally feel that is too soon but, by 75k or so it becomes a necessity. This will probably be between $1500 and $2000. The MB dealers in my area have no clue about this service, and it needs to be done by someone experienced, the bearing clearances are critical.
I don't know what you are planning to pay for this truck but, seriously consider a G500. They are out there with very low miles for $60k The milage will be the same, the power is worlds ahead. IMO a '98 G320 with 50k should be about $45k tops. There is a '99 G500 on ebay now with 38k and it is bid up to 40k. Make a good buy and keep it a long time.
