Is it 'safe' to use a 700 x 47c tyre on a rim with inner width of 15mm? Looking to upgrade my tyres on an old roadie mash up to something chunkier so I can handle some off road sections on a ride coming up, currently it has a 27 x 1 1/4 front tyre and a 27 x 1 3/8 rear with plenty of clearance all round*. 

I've got a set of 700c rims (internal width 15mm) that I am looking at putting on in place of what's already there because the tyres are cheaper for it, so would the wider 47c tyres be suitable for these rims? They have had a set of 32c tyres on them for ages no problems but it still looks like they'd fit a wider tyre quite fine, Just want to make sure first.

Also if wouldn't mind knowing what would be a minimum size tube to use, hoping to avoid having to buy new ones because I am on a $50 budget to get the bike suitable for the ride. At home I already have a number of thinner tubes that I doubt would work (up to 28c I believe) but also a pair of 27 x 1.25" tubes, Would these work for the 700 x 47c tyres? or would I need fatter tubes also?

*should be noted that the tyres already on the 27" rims are slicks or near enough and are falling apart due to being  left out in the weather for many years.
To fit 700c (ISO 622) wheels into a bike built for 27" (ISO 630) you only need to shift the rim brake pads down 4mm. If you're exceptionally lucky your current brakes will have enough reach and you'll just have to adjust the pads. That's the first thing to check, because if that doesn't work there's no way you're going to do this on a $50 budget.

2" wide tyres should be ok on the 700c rims. Not ideal, and probably outside the manufacturers specifications, but it should work. I've done that with 406 wheels before and it's worked pretty well.

The real problem is the tubes. Again, new tubes overstretched like that would work in a pinch, but they're be extra vulnerable to punctures because of the stretch. But usually if you're replacing aged tyres I'd suggest new tubes since they're likely to be in similar condition. Trying to over-stretch old tubes is very likely to fail immediately, and if not will almost certainly fail shortly after you start riding.

If you can get the 700c wheels to work, I'd go with a 38mm tyre rather than 42 or 48, just because I'm really wary of damaging the tubes. I assume on your budget you can't afford to carry a spare. But really, if you're doing a ride of any length I think you should buy at least one spare, unless it's a ride you can bail out of and get someone to carry you and your bike home. Even a cheap, shitty spare tube is better than nothing. personally I would cheap out on the tyres in order to buy a couple of tubes. It's not worth going out on a ride and having to go home because your old tubes give out.

If you can't make the brakes work I suggest buying the fattest 27" tyres that will fit your tubes, which is likely to be 1 3/8". I'm sure I've seen 1 1/2" x 27" tyres but I can't find any now.