Based on your research and the estimates you’ve received, the cost of 30-35k for labor and parts (excluding the ring piston, which you’re providing) seems reasonable for a full engine overhaul. However, the wide range of estimates you’ve found (35k to 1 lakh) could depend on the quality of parts, the mechanic’s expertise, and the extent of the repairs needed.
To avoid being overcharged, consider getting a third estimate from a trusted mechanic or garage. Also, ensure they provide a detailed breakdown of costs. It’s always better to clarify upfront to avoid surprises later.