If your broker does not support multi-leg orders then you are trading with one hand tied behind your back. A combination order order serves several purposes.
1) If your trade is executed, you will receive at least the net price specified and once in awhile, more.
2) You avoid leg in risk. When legging into positions, you may get a better or worse fill on the second leg.
3) Combo orders allow you to split the bids and ask for a better net execution price. You're likely to get some price improvement if the B/A spread on each leg isn't narrow.
The inexperienced should always enter the long leg first and then complete the combo order by selling the short leg which carries the risk. When you have sufficient experience AND you are a disciplined trader, if you are legging in, work the leg with the most premium and the widest B/A spread. IOW, if selling an ITM put for $2 with a 25 cent B/A spread and buying and OTM put for 20 cents with a 5 cent wide B/A spread, there's little to no price improvement available from the 20 cent put.
Your question about risk is a bit confusing. The more ITM the short put is in your bull vertical put spread, the larger the credit and lower the total risk of the position. However, the deeper ITM it is, the larger the delta and therefore the higher the probability that the option will expire ITM (delta is an approximation of the probability of the option expiring ITM).
Think of it this way. Suppose XYZ is $50 and the Oct $60 call has a delta of 5. That means that it has a 5% chance of being ITM at expiration. Not a good bet, eh? It's cohort, the Oct $60 put will have a delta of 95 or a 95% chance of being ITM at expiration. However, this doesn't address probability of profit (POP) or expected return and the latter gets into more complex math.
For POP, read the simplified POP explanation at tastytrade http://tastytradenetwork.squarespace.com/tt/blog/probability-of-profit
For a simple estimate of the trade probability use the free "Probability Calculator Software" at http://www.optionstrategist.com/calculators/probability .
This may sound confusing but if you want a really lazy way to calculate the probability of your spread, calculate the delta of a hypothetical put (same series) with a strike price of the break even point of your spread. It will provide a ball park result near that of the above calculator.