Hi Maxm.
Welcome to our Mattress Forum! 
These are all reasonable considerations. You are heading in the right direction!
With regards to your weight and sleep styles, I would suggest (if you haven’t already) reading our Mattress Durability Guidelines and also our article on the various sleeping positions and the support needs each one requires.
Lower back pains can be often an indication of foam breakdown and mattress sag compromising a neutral spinal alignment. Soft foams in mattresses are also breaking down much faster then firmer ones. You’ll definitely be able to find something within your budget that meets both you and your wife’s needs – although you’ll need to speak directly with the various mattress companies to find out if they offer any sort of removal or recycling services for your old mattress and boxspring.
Provided that you chose the correct support/comfort balance for you and your wife are all good choices, the various mattress companies you mentioned in the title of the post all allow for in-home sleep trials, and returns (though not all will refund shipping, etc. so make sure to read those policies carefully).
My Green Mattress, Gardner, GhostBed , and SleepEZ are all Trusted
Members of our forum whom we think highly of (like all of our Trusted Members and that I believe that they compete well with the best in the industry in terms of in terms of their quality, value, service, knowledge, and transparency.
My Green Mattress and SleepEZ also have dedicated forums here where you can ask them direct questions about their products and policies directly.
I highly recommend you to rely on the advice of the manufacturer themselves as they are much more familiar with their own mattress designs and materials than anyone else (including me) and they can use the information you provide them about your body type and sleeping positions, your preferences, your history on different mattresses, and the results of your local testing to make suggestions based on the “averages” of other customers that may be similar to you. The more accurate and detailed the information you provide them the better you will help them to help you make the best possible choices out of the options they have available.
mdgail1 recommended a good solution with a side-by-side split option to mitigate the differences in sleeping positions between you and your wife. This will also give you several layer arrangement possibilities to experiment with and fine-tune in-home the mattress support/comfort.
If you sleep prone, as you may be aware, this is the worst posture for your lumbar region, regardless of core strength. Everyone needs good deep support to assist with alignment. Your muscles relax a bit when sleeping at night, so maintenance of alignment falls mostly to the mattress itself. You may wish to review the guidelines in this [url=Your sleeping style, preferences, and statistics - Your sleep positions - The Mattress Underground] Sleep Positions Article [url] so that you avoid hyperextension in a swayback position that can cause back issues. Stomach sleeping is a common source of back issues as stomach sleepers tend to sleep in a “swayback” position, especially if their comfort layer is too thick and/or their support layer is too soft.
Generally, I’d recommend you see the mattress shopping guidelines … make sure to avoid major brands or any mattress that don’t disclose the quality of the materials inside them because there is no way to identify any weak links in a mattress or make meaningful comparisons with other mattresses. (In most cases, the major brands use some low-quality materials in the upper layers of their mattresses that make them a risky choice in terms of durability and the useful life of the mattress). Saatva, on the other hand, provides all the information you need to make an informed choice in terms of the quality/durability of their materials.
You can read a detailed analysis on this Saatva mattress thread.
I look forward to hearing what your finalist(s) are … hoping you don’t face the same issues with posting this time around!
BTW … I remember in the past another subscriber trying tp post and having the same issue. We narrowed it down to an emoji being pasted and the system would give an error. As we recently changed the forum template there may be a similar issue. If the issue occurs with your next post.I’d be very grateful if you could report an issue here for our Dev Team to have a look at.
Phoenix