Buying a rigid vang is a great idea, getting rid of the topping lift is also something I like as it cleans things up and reduces chafe on the leach of the main. My spinlock rigid vang has a ratchet switch that allows the boom to be rigidly supported during reefs and when the sail is down. I don't know if the Gerhauer model does. If not you may have to deal with the boom dropping during the reefing process (not a real problem, just realize that you need to drop things gently or you might lose some gelcoat) This is the one advantage of a topping lift.
Does the Gerhauer come with mast/boom fittings? If so I'd expect they are engineered for the expected loads. If not and you have to buy/fabricate your own, then you do need to be sure that these fittings are up to par (and, of course, that they match the end fittings on the vang). For convenience and safety the vang should be led aft.
With that thought, perhaps you can use the currently led-back clew line's hardware to handle the vang and revert to a boom-mounted system for the reef line, since you are at the mast already.
If your boom-end and gooseneck fittings do not already have the required sheaves (btw- how is your outhaul set up?) then you can run lines inside from the cheekblocks with standard exit slots like you see for internal halyards..plus another exit forward to get to the cleats. This does require cutting into the boom, and if that makes you nervous then perhaps you're best to leave things external.
Depending on the boom section size, you can run your reefing lines down the same side or one on each side (more common). Run from a padeye (or around the boom if loose footed) to the clew cringle, down to the cheek block (opposite side of padeye if used) and forward to the cleat near the gooseneck. If you run each reef down either side, then you kind of have a favoured tack (windward side) on which to put each reef in. That takes a wee bit more planning and forethought.
Many booms have sheaves and clutches built into the gooseneck fitting; this works well too, and a single winch mounted on the underside of the boom can help with tensioning either clew if necessary. But it doesn't sound like you have that setup.
Check out the various catalogues and on-line resources for diagrams and hardware ratings.
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)
1984 Fast/Nicholson 345 "FastForward"