engine room should be about amidships, or at 2/3's from the bow.
you have no problem with longitudinal stability, but with transverse stability, the famous 'would float bottom up' problem. that radar antenna is huge. any radar antenna is huge. most frigates are powered by gas turbines, which are rather light.
ship designs are always an ballance of how high you can mount your radars (radar horizon) and how many guns and missiles. from the weapons, with the missile silo's (or an VLS system, as that is mostly called on ships) that high in your hull, your CoG will be about the height of your deck...... with all stabilty issues that come from that. the guns are even higher.... now you see, you need ballast in your hull to keep it upright. ballast that is unused space and weight on your limited displacement.
in short, for now: place everything as low as you can make it fit. to help you a bit:
viewtopic.php?f=16&t=2755
in this thread you can see what space systems need belowdeck. puzzle with them, try out...... I can give you this already: the mk 41 VLS is mostly mounted with it's bottom near the waterline.