As has been said, it all depends on why the airplane is VFR limited, and how much work you will have to do. Unless it is missing one of the IFR required components listed in FAR 91.205 which I have quoted below in part:
Add Day and Night Instrument PLUS
(2) Two-way radio communication and navigation equipment suitable for the route to be flown.
(3) Gyroscopic rate-of-turn indicator
(4) Slip-skid indicator.
(5) Sensitive altimeter adjustable for barometric pressure.
(6) A clock displaying hours, minutes, and seconds with a sweep-second pointer or digital presentation.
(7) Generator or alternator of adequate capacity.
(8) Gyroscopic pitch and bank indicator (artificial horizon).
(9) Gyroscopic direction indicator (directional gyro or equivalent).
I suspect that the VFR 150s have most if not all of this, so you probably need a static check, and maybe the clock will need to be repaired and/or replaced. BTW, if the clock or other instrument is inoperative, even for VFR flight the aircraft is unairworthy, unless the item has been placarded, and deactivated.
The more important question is why are all these 150s not IFR, and the answer is that long, long ago, almost everyone concluded that the C150 was too light and unstable to make a good instrument platform. Before you invest a lot of money on a C150 getting it IFR certified, go take an lesson or two in 150/152 (a VFR only airplane is just fine for taking instruction, you just can't file IFR), and then take a lesson or two in a 172. If you are still convinced that you want a 150 for instrument training, then pick the one that you want, that has the best complement of required instruments and radios, and take it to a radio shop for a quote. That is the only way that you will know for certain.