Is there an antenna that can be tuned to function from 160m to 23cm w/o needing to buy seperate units?
No. And if there were, it probably wouldn't work very well.
The same goes for radios in most cases.
Ham radio offers a wide range of capabilities and experiences, depending on your interests.
And because everyone's interests are different, there is no "right" answer for anyone.
(Keep this in mind - lots of folks will assume that your interests are the same as theirs,
and will try to convince you that the rig they choose is best for you. Much of the time
it won't be.)
But that also means that spending some time to think about what aspect of ham radio
appeals most to you will help us to give you some suggestions.
For local communications, most hams start on 2m FM, as there are local repeaters in
most areas that will provide coverage out to 20 - 30 miles or more, often with a
hand-held radio. This is convenient to chat with other hams. In some parts of the
country the 440 band is heavily used, but in rural areas there might not be any active
repeaters. Where you live will determine whether this is a useful addition to your
capability - you will need to see what linked repeaters are available in your area.
To work further on VHF/UHF requires better antennas, and especially more height.
This might be temporary, but even an antenna in the attic, or taped to the inside
of an upstairs window, will work better than one in the basement. Real long
distances also would use SSB or CW modes instead of FM (which is used for
repeaters), but that is another whole area of interest.
If your primary interest is repeaters, then for most people having 2m and 440 is
quite sufficient. There are repeaters on 6m and 10m, but propagation is much more
sporadic. Again, it depends where you live. Some areas have repeaters on the
higher bands, but they tend to be sparse and equipment is not as easy to find.
While a hand-held is convenient, they have a lot of limitations, including battery
life, cramped controls, displays that can be difficult to read, and a tendency to get
rather warm in the hand after extended operation at high power. While I have
several, I rarely use them. Instead I have a mobile 2m/440 rig connected to a
12V power supply under the desk as my base station. (This also give you the
ability to run it off the car battery for portable operation.) This is probably the
most common setup - it provides good local communications and I can hit
repeaters up to 100 miles away - but that's because I'm on a hill and have an
outside antenna: typical ranges are less than that.)
Most mobile FM radios have a low power position - I run mine at 5 watts most
of the time, only switching to high power when needed for a specific situation.
That's pretty rare - but, then again, I have the advantage of height. I wouldn't
recommend adding an external amplifier to such a rig until you've done everything
possible to improve your antenna.
The HF bands (including 160m, which is actually MF) will cover longer distances,
and require longer antennas. There are a few repeaters on 10m, some of which
may be lined to local repeaters, but they generally aren't connected to EchoLink
or other systems because signals will fade in and out due to conditions. I like
having separate rigs for 2m FM and HF so I can listen on one while talking on
the other, but others like the all-in-one-box approach. It is possible to talk
around the world with an antenna hidden in your attic, or wires strung across
the roof, but the again the higher you can get your antenna, the more effective
it will be. More commonly such antennas should allow you to make contacts
around much of the country, and occasionally further. The "standard" 100W
HF rig is a good start: some of use like to use low power (5 watts), others
run the full 1.5kW output (which I wouldn't recommend with an antenna in
the attic.) Again, you will discover what bands you like to operate, whether
you prefer to work DX (distant stations) or chat with friends out to a couple
hundred miles, etc.
So my suggestion would be to start with a FM mobile rig, either 2m or 2m/440,
depending on the level of 440 activity in your area and the frequencies of
the linked repeaters near you. Get up some sort of antenna - not necessarily
a commercial one - that provides enough signal to hit the repeater reliably.
This shouldn't cost a lot - a few hundred dollars perhaps, including a power
supply to provide operating voltage in the house. If you expect to use it while
you are walking around, then an HT may be better, but the mobile rigs are
much more convenient for operating from a desk.
Then join the local radio club and ask the members what their interests are
in ham radio. Maybe visit their shacks to see what equipment they use and
ask why they chose that particular model. See what others use for antennas,
especially in constrained situations. You may find yourself fascinated by APRS,
working DX on 160m, EME, QRP, building your own equipment, contesting,
emergency service, radio direction-finding, traffic nets, antenna design, or
any of a number of other options. As you do, you can focus on extending
your equipment to suite those interests.
You don't have to start with everything all at once - it gets overwhelming.