Wanting an FMC to manage speed and altitudes is a big ask (!), as this is certainly moving into payware territory of better aircraft. VNAV is one of the hardest components of aircraft avionics development to get right.
The Honeywell FMC is superb, and it does from memory manage advisory VNAV (not used it for ages, however).
VASfmc is another example, as not only does it give users a full MCDU, but you also get MCP panels and PFD/ND's too. It's freeware, and sadly it doesn't work with all versions of FSX. It's Navigraph friendly, and runs in Windows (NOT in sim).http://c-aviation.net/vasfmc-freeware-fmc-for-fsx/
There are some excellent payware gauges/FMC's, but the one that springs to mind are those by Ernie Alston ISGsim Simavionics, giving you advanced features for reasonable cost; the main drawback is that this will not interact with the native displays of your aircraft, so you'll need to edit panel.cfg in order to place the FMC's and PFD/ND's into the VC (or 2D) panels. However, a number of preconfigured cfg files for a large number of aircraft are available. http://www.isgsim.com/?page=home
There are many others, including Eric Marciano's EasyFMC (http://emarciano.free.fr/En/EasyFMC.htm
) though I'm not sure if this one allows speed/alt constraints, but the above two are worth checking out.
Another point to consider is whether such gauges actually allow you to enter flight plans from scratch, or whether they only allow you to load the presently loaded FSX flight plan. Equally, some but not all will allow you to edit routes OR insert other waypoints, or enter direct to's that are not already present in the flight plan.
It may be worthwhile adding FMC's to older aeroplane, so that they get a new lease of life, for instance, CLS and Just Flight's "F-Lite" range can be brought up to date with ISG gauges.
On the other hand, what you are looking for is full functionality and that is hard to add into an existing aeroplane after it leaves the developer; retrospective additions are good but not perfect!
TBH, I would heartily recommend looking for a decent payware aircraft during the sales, such as QualityWings 757 or 146, or perhaps Captain Sim. Don't scoff at my CS suggestion, as let me tell you that I had a great flight in my CS757 last night, and it was faultless in LNAV/VNAV, and once a year CS has a one day sale - $9.99!
Equally, look for SimpleFMC, as this little Windows utility can drive a non-FNC equipped aircraft (737-200, DC-9 etc) and if you boost it's now old navdata with up to date fixes, NDB's, VOR's (easily done with Excel), then by simply entering a fix (or list of them), you get instant heading-hold driven LNAV.