I owned a 1998 Blazer (basically an S10) and I currently own a 2003 S10 4.3
How reliable are these trucks? very reliable if maintained however there is some common failure points.
1. Intake Manifold Gaskets (4.3)
They tend to fail around the 100,000 mile range which basically acts like a head gasket leak this is due to DexCool which most swap out for conventional green but you must be sure to thoroughly flush the system if you go with conventional green. I personally went with a universal coolant in mine.
2. Fuel Pump
The fuel pumps on these trucks and many GM vehicles of this era fail around the 100,000 mile range usually they give a warning sign first in terms of loud whining. Mine started failing at 122,000 miles and eventually started giving first start issues I then replaced it with a US Motor Works fuel pump and am nearing 124,000 miles now.
3. Distributor
The distributor on these trucks and again many GM vehicles of this era tend to fail around the 100,000 mile range, I personally swapped mine out for a Duralast Gold no problems out of it.
Other than that the rest is just your typical normal wear and tear items. You do want to make sure the transmission has been maintained. If the fluid is very dark or smells burnt walk away unless you want to put around $2,000 for a transmission.
The 2003 has the better engine assuming it is a 4.3 and not a 2.2, it is a LU3 4.3 these have an updated injection system the 1996 that RayVoy had in his and in the 1998 Blazer I had was inferior to the 2002+ LU3 engines. The LU3 also has 190HP vs the 175-180HP the 1996 and 1998 had. It is a smoother engine but still really all of the 4.3's are very tough engines and can last a very long time if maintained.
My truck is in very good condition with a lot less miles and cost me $3,000 so I would offer $2,000 if the truck is solid. Be sure to have a good mechanic check it out before you buy it to make sure it is solid.