I just bought a 2001 E350 that sat for the last 4 years and has a couple issues. The. Speedo doesnt work at first but does after several miles. The odo and trip seem to work fine even when speedo doesnt. The front fan doesnt work but rear does. Going to replace the switch. Cat was stolen but I replaced it with a straight pipe, will I have any problems by not replacing the Cat? Any thoughts on speedo would be welcome.

 

You might get annoyed with a check engine light because the system does use info from the sensor after the converter to control some fuel trim actions.
The bad part is, if there is any other system problem you won't know because the CE light is always on.
Might be a good idea to check the fuses, there may be one for the dash gauges.