One of the aircraft I fly, has for a club aircraft, a decent amount of nav kit – ADF, DME and a glideslope localiser. It also has a Garmin GPS 150 which I have to say I find very usefull and not just for NDB approaches.
Unfortunatly its not capabable of performing GPS approaches and the cost to upgrade to say a avidyne IFD440 would I guess in the order of 15 grand.
Is there any market for a simple GPS only unit?
A KLN94 would do this. It is what I have. The moving map is nearly useless (very small) but a real pilot uses an Ipad for situational awareness doesn’t need that 
One problem is that you can’t buy one on US Ebay because you need an EASA-1 form with any used avionics installed in an EU-reg plane. So you would need to get one “processed” by someone over here – even if buying it from a European source, or just install it off the books (it is old enough to look like it went in many years ago).
A GNS430 is also fairly cheap now, 430W a bit more, but the moving map on these (and the IFD440 / GTN650) is still very small to be your only moving map, IMHO, especially for predominantly OCAS flight which is, for practical purposes, in the UK, “VFR”.
Do be aware of the fundamental difference between KLN94 and GNS430 on the one hand, and the GNS430W and GTN (and, if you must, IFD440) on the other.
The former is certified to TSO C129 and does not support 3D approaches or angular navigation (all other approaches are angular, meaning increasing sensitivity as you approach the runway)
The latter are certified to TSO C146 and can do LPV approaches to 200’ minima.
Whichever you install, it must be certified and approved in the Aircraft Flight Manual for approaches.
I agree. Get an SBAS unit these days. We’re in 2018.
Regarding the question: I don’t think there is a market for a GPS-only units nowadays. I mean, that’s what the markets shows. Everybody who flies IFR also needs proper ILS and proper COMM equpiment (not the ancient KX155 stuff etc.). Add to that the 8.33 saga. Every aircraft will need dual 8.33s eventually. So that’s how one kills two birds with one stone.
If you really want one (a GPS-only box) and SBAS, there is the (rare) Garmin GPS 400W out there on the used market. The KLN94 is REALLY ancient, doesn’t contain a lot of procedures, etc.
Sure, this is why a GNS430 still fetches a good price. You get an 8.33 radio with it.
In the UK at least (where the OP is) there are almost no LPV approaches, and probably most of these boxes get installed “off the books”. I know one installer who spent most of his life installing these, never doing any paperwork. What’s an AFMS?
The KLN94 seems to contain all GPS LNAV approaches. I have not seen a missing one yet. It doesn’t contain RNAV SIDs/STARs but does contain their waypoints 
The OP needs to judge this in his very specific context which I suspect is a UK PPL school.
Peter wrote:
In the UK at least (where the OP is) there are almost no LPV approaches
… yet.
If anyone is interested in a GNS530W, I had mine just removed for a GTN750/G500TXi/G5 upgrade. PM me for details.
Bathman wrote:
Unfortunatly its not capabable of performing GPS approaches and the cost to upgrade to say a avidyne IFD440 would I guess in the order of 15 grand.
In theory, you can get a GNS430 and upgrade it later to WAAS. Or you can get a 430W right away (which I’d recommend) or then go the GTN or IFD route.
Installation cost depends hugely what goes with it, AP integration, HSI / Aspen/ e.t.c. integration and so on. But I would think installation will be around the same ballpark for any of these. Unit costs are widely different, GNS430 plain can be had around 4000$, 430W go for between 6 and 8k$. The question is if it is not better to get a later unit for the difference rather than upgrading again in a few months.
But for your original question, a GNS430 will do most of what you want and get you 8.33 Com and it IS upgradable. What I’d do is to make sure it has a WAAS approved antenna and cable from the start, then the actual upgrade to WAAS is simply a plug and play thing.
I can’t say what approvals would be needed for outside US or N registration. Here is a link to the latest GNS530W AFMS
