Hello,
Cessna 177 is an excellent, roomy airplane, but much better if you aren't the person paying to fix it! If it's an RG, the landing gear is really cool, but fussy, and expensive to maintain. Though it seems to work the same way as the 172 and 182 RG, there are many differences which make it less desirable. Also consider the cost of fixing minor damage, it's not common plane, and getting old. Many airframe parts for the 177 are getting very hard to find as replacements. Insurance companies know this, and will price hull insurance accordingly. The more common the type of aircraft, the more available, and thus more economical, the replacement parts.
The 177 could be operated from 500m by a pilot with experience on type, and favourable conditions , but planning this type of operation as a new venture may be expecting too much, particularly with passengers. A STOL kit, and really good training from an instructor very familiar with the 177 and STOL kit flying would be a benefit.
A Cessna 182 would be a better choice than the 177 if this load out of this runway length is important.
172XP is a great plane, and reasonable cost to maintain, other than the TCM IO-360 engine, which can incur some expensive repairs over time.
The Cessna 180/185 would be an excellent choice for these requirements, but more pilot skill requirements, and higher insurance costs will be involved.
The Piper Cherokee 235 (Dakota) would be suitable, other than it may not leave you feeling relaxed in a 500m runway. Straight tail Arrow is also a great plane, but avoid the "T" tail versions into short runways.
You could also consider the Found FBA-2C series of aircraft, they would meet all of these requirements, with the exception Mogas, and possibly autopilot.
120kts cruise speed is going to cost you quite a bit more. Do you really want to pay half again the cost for that added 10-15 kts?
Cheers, Pilot DAR