MH is passé - so you should be able to get some good deals on MH systems BUT they are $$ to run as bulb must be replaced every 6-12months (spectrum & energy degrade much more than with T5's etc, LED are much more stable & cheapest to run long term) & in some areas you'll be limited by bulb availability (make sure you've a good online source with cheap shipping etc - I can't get cheap shipping so I always suggest this be part of the budget)
If tank was a more standard height 14-16in, lighting can be done cheaply & well within your initial budget - you can always run the tank with a 14in water column (measure from substrate surface) & add in a mister for an interesting upper tank half BUT if you include many submerse plants, they will quickly block light to substrate plants
A "wall" effect of submerse plants kept trimmed bonsai style could work well (though this setup needs to be added into the budget as well then)
I like deep (tall) tanks as I think most fish prefer less shallow water columns (look at natural habitats & even those considered "shallow" are at least 18" deep).
Note that many of the T5 systems on the market are very poorly designed - reflector material & shape is poor, bulbs placed much too close etc
4 HOT5 properly spaced across your 18" tank width & good reflectors can supply sufficient light to grow most plants, including MC (HC would take some special consideration
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BUT the 60" length is an "odd" so look at costs of running those T5's - you want HO & proper spectrum on these bulbs due to the water column height, I had considered a 60in tank some years ago but couldn't source bulbs locally (& the light unit was going to cost double what I could get on a 72" unit)
3 Kessil A360 would provide a nice clean look, check the classified section here, there were a couple for sale but make sure of warrany transfer etc & consider if you're really getting a deal should a light fail
The tank would also look very good with just 2 A360 to start, adding the third next year etc - or you might get a bit of a discount by purchasing all 3 ... you could do a light at each end & a shaded area in the middle etc
This would also reduce plant start up costs