You have a Suntour 5 speed freewheel (threaded) on what could be a 120 mm or more likely 126mm frame spacing and will need a 4 prong Suntour tool to remove this freewheel... the threads on the hub will take a threaded single freewheel with no issues and if you need a good tensioner just use your old deraileur (if it is sound) as there is no better tensioning device ever made.

The limiter screws on the deraileur can be used to lock in it's position.

On a 5 speed the spacing the chain line is going to come out pretty nicely if you run your chain on the middle chain ring position and will probably be nearly exact.

I would go back the shop and ask for a refund since they did nothing right.

More details about your bike are needed... don't know what the front chain rings are and what kind of gearing you are looking for... a 52:18 makes for a very smooth and long running drive and yields a gearing in the low 70 gear inch range.