you will be much better off with the 150x750 down the trk as opposed to the 4se [you are paying a lot for the goto mount]-the 6" f5 is capable of seeing planets, dso's & taking pics if you decide to go that way
you are probably better to buy either the OTA itself -and look for an EQ mount her in the classifieds -upgrading from a standard EQ to a goto EQ is usually not worth it -the conversion kit is often ore than secondhand complete goto mount
there is 1 mount in the classifieds for 450
LXD 75 it would go with the 6" newt OTA fine, but this
mount for 800 is a lot better