Judging from your post, you and your wife have different requirements, but requirements which can be handled by one type of machine.
A typical SBDU (Single Boiler Dual Use) machine would be perfect for making those straight espresso shots, but your wife may find that making latte's with a SBDU can become tedious.
When I had my Rancilio Silvia, I found that it took me 10 minutes to prepare a drink, from start to finish. That included temperature surfing, brewing, post-shot cleaning, waiting for steam, steaming, post-steaming cleaning and flushing for cooldown and boiler refill. You yourself may find that temperature surfing your straight shots to be tedious after a while.
Since you don't have company over often, it isn't a big deal and a SBDU may suit your needs just fine. My only fear is that your wife may get upgradeitis and demand that you consider purchasing a heat exchanging machine when she doesn't want to wait to make her lattes.
Ideally, for your needs, I would say that a low end Heat Exchanging machine with a reservoir and a vibration pump is probably the best machine for your needs and your wifes needs. Unfortunately, it will blow your budget out of the water. If you want to make espresso NOW with the budget that you have, then a used SBDU is your only option.
Now, since you don't mind buying used, I would consider getting your hands on a used Nuova Simonelli Oscar. They sell for roughly around $500-ish. I realize that is more than what your budget allows, but the Oscar is a very different class of machine over a typical SBDU and may stave off the upgradeitis (and complaints ... :-) ) for a while.
Garbage In, Garbage Out, for every step of the process. From Beans to grinder, grounds to machine, coffee to cup.
With my lack of budget, I'm going to spend what I have now on a high quality dual purpose grinder and save up for a proper espresso machine that will suit both my wife and myself. In the meantime, I will keep an eye out for a good used machine and purchase if the opportunity arises.
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