I wouldn't replace the downdraft with an overhead RECIRCULAING hood - if you can vent the overhead hood that would be the best. As far as options for a new downdraft, I know others have had success with telescoping ones, but I don't know what's required vs. what you have in place - others may be able to help here.

>>>"If we do end up replacing the countertop, will we have problems installing a 36 inch cooktop in a 48 inch cabinet? #1 on my list for induction is the 36" Wolf"<<< Not a problem with a 48" base cabinet. Actually, your choice of a Wolf cooktop avoids problems even if you later decide to replace that 48" base cabinet with a 36" one. IIRC, Wolf thoughtfully designed their cooktops to need cut-outs a bit less than 34½" inches. (That is for the subsurface box containing the burner components, control electronics and mechanicals like fans.) Other brands are a much tighter fit if they aren't too wide altogether for a standard 36" base cabinet. Remember that the 36" measurement is the outside dimension; the interior space is diminished by the thickness of the side walls which, depending on what the cabinet is made from, will be something between 1¼ and 1½" narrower than the outside dimension. That's the subject of the discussion at theinductionsite.com that I referenced above.) But, maybe, your concern was weight of very-full extremely large pots on the cooktop? Are you planning on boiling 10 gallons of wort in a huge brewing kettle? Making 10 gallons of pasta at a time? Frankly, I'd be more concerned about the weight-carrying limitations of the cooktop's ceram/glass surface. (Some cooktop's use and care guides caution against pots which, with contents, weigh more than 50 pounds.) Of course, if you need "belt-and-suspenders" reassurance, you just add a couple cross braces at the top of the cabinet on either side of the cutout. >>>"I think I would prefer frameless, but in another thread I read that it's harder to lift out that cooktop if we were to replace our cabinets later." <<< Why lift out the cooktop when will you will be immediately putting those countertops back on top of the new base cabinets when and if you swap the base cabinets? We've always left cooktops and sinks in the counters when lifting countertops that we intend to put back on. It might l be a different matter with a pop-up downdraft vent. (Yes, you will want venting even with induction.) The one time I had to work on telecoping downdraft vent, it was pretty easy to disconnect the countertop=portion from the rest of the unit. That may not be true of all of them. Something to check on when shopping. >>>"we're just trying to make the right decision as far as timing goes."<<< I'd be inclined to replace laminate countertops now, all other things being equal. Laminate counters are very serviceable without being expensive. Generally, laminate is pretty easy to work with. Usually not expensive if you need/want custom sizes or counter shapes. Can do a lot to pep up a well used kitchen. Also, much easier to install a new cooktop and telescoping downdraft in a new counter than trying to fabricate a temporary set-up over the old, longer but narrower cut-out. (IIRC. the old Jenn Airs had two 3 to 4 inch vent grates in between the 3 pairs of burners, which is what resulted in the odd length of the unit. An add-on downdraft will need to sit at the back of the cooktop, and you thus wind up needing a very different cutout. BTW, have you seen the threads from the last year or two here where there were discussions of how to install a range hood to connect with the old down-draft vent's ducting?

Basically remove all the down draft venting that's in the way and seal up the vent. If you put in a over the range hood you can us a recirculating hood since you most likely do not have a vent pipe above the range.

If you have to have a downdraft, you are infinitely better off with a nice induction (not to be confused with smooth top radiant electric.) Induction allows for ease of control like gas or better and the easiest clean up ever, since nothing but the pan gets hot, not the cooking surface. Induction is a cleaner technology, so there is less stuff to suck out of the air and you won't have the flames of the gas hobs being affected by the suction.