There are a few things you want to take into consideration.
1. Purchasing the land - Most lenders do not offer loans for purchasing land. The reason for this is that land, in most cases, is harder to flip if they have to foreclose on it than an actual residential property. So this would mean that you would have to purchase the land in cash.
Not necessarily something I would recommend to a first time buyer.
2. Lenders that do offer lot loans will require that you bring 20% to the table. So, for example, the lot costs $150k, your 20% would be $30k.
3. Interest rate - You should also expect your interest rate on the financed portion to be at least 1-1.5% higher than the market interest rate at the time. So, if market rate is 6.5 on a single family residence, your interest on the lot (if financed) would be anywhere between 7.5-8%. And that is assuming you have good credit.
4. Cash for construction - the typical builder is going to look to you to provide the cash to provide the various materials during the course of construction. This is broken down into “draws” or phases.
Let’s assume the cost of construction is $250k, you may have 5 draws at $50k each. Again, doing a custom build requires you to have a good bit of expendable cash on hand. Not typical for first time buyers.
5. Best advice I could give you as a first time buyer - go the new construction route, as in, find a community that is being developed by a reputable builder and purchase a home from them (for example, D.R. Horton, Meritage Homes, Pulte, etc).
These large builders are able to build at a lower cost than smaller builders due to the fact that they are building a large number of spec homes at a time. When they hire a concrete sub contractor, they have them out in a community pouring the slab for a number of properties, not just one. So it becomes more cost effective.
Most of these big builders will also offer incentives like money towards your closing costs, lower interest rates, and potential upgrades. Much easier process. And much more cost effective for a first time buyer.
Hope that helps.