1. If I go with ladder line, how long should the ladder line be with an apex about 45 feet?
If you are trying to recreate G5RV, then use about 32' of ladder line times the
velocity factor. (Probably around 28'.) Looking at VK1OD's analysis here:http://vk1od.net/antenna/G5RV/index.htm
the SWR on the coax and associated losses should be reasonable on 80m,
40m and 20m (though the impedances will be somewhat different with an
extreme slope to the antenna.)
Otherwise, the ladder line should be long enough to reach to your tuner, or as
close as you an get it. Occasionally you may encounter an impedance that your
tuner won't match due to a particularly combination of antenna and feeldine
length, but you deal with those cases individually when they come up by adding
feedline, switching a coil across the balanced line, etc.
2. I have a MFJ 949D turner with a balanced connection, with a 4:1 built in balun, should I go straight into that with the ladder? (I am concerned with TVI, lots of wiring in the home)
Running straight to the balanced output on the tuner is fine. (Don't forget to add
the shorting wire from the single wire output jack to the balun.) Radiation from
properly balanced open wire line is quite small - less than what you can have on
the shield of coax cable with many antennas. If you run one side of the ladder
line against the case of a computer or TV it will pick up more signal, but there
is a simple fix - don't do it.
Otherwise running ladder line into the shack
is no more likely to cause TVI than any other feedline.
Your primary cause of RF interference is fundamental overload due to the
transmitted signal from the antenna being stronger than the equipment can
handle - that isn't the fault of your feedline or your transmitter.
3. Would it be better to come off the ladder line with a short coax, about 10' length connected to a balun "outside" the window and come into the turner with coax, straight to the coax input on the turner.
The length of the coax between the external balun and the tuner is
generally more important than the length of ladder line from the balun to
the antenna because the losses in the coax will often exceed those it the
rest of the system combined. The ideal coax length is generally "as short
as possible", preferably measured in inches rather than feet.