I own a '93 Chevy 6.5TD myself, which happens to be my first diesel as well. Additionally, I am fairly familiar with some of the other brands of diesels and their good and bad points. My dad has a '97 Dodge with a Cummins in it. The GM 6.5L trucks are generally more affordable to purchase than some of the other brands of diesel trucks. You can put a lot of miles on them if you know how to take care of them and what to look for. They're still putting 6.5L's in the military HMMV's so that tells you something. There are some things to keep in mind with this engine, however.
'93 and prior versions do not have the PMD system and use a mechanical fuel injection system instead. The '94 and later models of the 6.5L are known to have problems with the PMD for the injection system. The electronics just aren't robust enough to take the high heat location and unability to dissipate the heat where the PMD is mounted from the factory. If you want to keep the truck, I'd get a PMD isolator kit from somebody like Heath Diesel and that should eliminate your problems.
http://www.heathdiesel.com/T/Chevy-GM/Chevy-9495/PMD-Isolators/
The lift pump is a normal maintenance item with any vehicle that has one and they're easy to replace with the 6.5L ( at least on my '93). The lift pump on the 6.5L is relatively cheap compared to others. I got a new one from the GM dealer for around $150 and you can find some off brands cheaper. Some of the Dodge in-tank lift pumps will cost you $600-$700 to replace from what I've read.
If you've replaced the lift pump, I'd highly recommend replacing the oil pressure switch that powers the lift pump when the engine is running. It's only about $30 from GM and will make sure your lift pump is powered, has the proper voltage, and is working to keep the expensive high pressure injection pump fueled up and properly lubricated with fuel. You can get a crow's foot oil pressure switch wrench from Snap-on for around $30 to replace it if you want to do it yourself ( it's in a tight spot under the intake manifold ).
If the high pressure injection pump gets starved for fuel, you'll start causing excessive wear and start to damage the injection pump and the truck may not run properly. A faulty oil pressure switch that's been missed could be the reason you've replaced so many PMD's if the PMD tends to get blamed when there's a fuel related performance problem. For that matter if there's generally low oil pressure, that may not be actuating the oil pressure switch properly. I installed a fuel pressure gauge in mine to to monitor lift pump pressure and make sure it's working properly.
Batteries and cables go bad with any vehicle, and so do starters, so I wouldn't hold that against the 6.5L unless it's got low miles on it. You may have the starter solenoid contacts going bad, causing high resistance and low voltage to your starter. Might be as simple as replacing the starter solenoid contacts or just getting a re-built starter depending on what's more cost effective.
If you want to read up on the GM diesels, I'd highly recommend reviewing "The Diesel Page" It's the best resource I've found for the 6.5L diesel.
http://www.62-65-dieselpage.com/
I've bought both their books that have collections of articles on the 6.5L and they're very informative. They will probably more than pay for themselves, especially if you do any of your own repairs.
So, other than the PMD problem, I wouldn't be too critical of the 6.5L. If you have further questions that I can help with, don't hesitate to post here.
( I am not affiliated with any of the sources listed here in any way. )
